(1) The sign of gender or sex, like male, female, neuter (in Hindi Pulling, Stree Ling).
(2) Male sexual organ of generation.
(3) Western commentators use this concept of male generative organ as the basis for their interpretation, for example Monier-Williams dictionary provides one explanation of Ling as "the Male organ or Phallus (especially that of Shiv worshipped in the form of stone or marble column which generally rise out of "Yoni" (female generation organ), and is set up in the temple of Shiv". They have combined the two meanings claiming that Hindu represent their God by male organ and are phallus-worshippers. There was and is, truly no such connection in the minds of Hindu people. (see also below Other Interpretations)

In fact they consider Ling as the flame of sacred fire and its base Yoni as the Vedee or Havan Kund. In the Shiv Puraan, Shiv is made to say - "Although I am omnipresent, I am especially present in 12 Ling, where I am called by different names". These Ling are situated in 12 different cities and are called Jyotirling. (see their list "Now Back to Jyotirling" and read about them under Shiv Temples)

(4) Also called Shiv Ling, Ling means mark sign or mark in Sanskrit [this meaning is used in Geetaa, Chapter 14, Verse 21]. This is a symbol to worship Shiv. It is a short cylindrical stone with semispherical top in shape. While its origin is debated, it dates back to Indua Valley civilization. Vaman Shivram Apte's Dictionary gives 17 meanings of Ling, including, the image of God, spot or stain, a symptom or mark of disease, etc etc. An example of the use of sign occurs in Bhagvad-Geetaa - "kair lingais trin gunaan etaan, ateeto bhavati prabho". This is translated as "By what marks is He recognized who has transcended the three qualities, O Lord?" It is the most prevalent icon of Shiv - usually a short pillar with rounded top, usually set on a circular base or "Peeth". The "Ling" word, as a name for Shiv, appears in "Shiv Sahastra Naam", where it is translated as "a symbol of the origin of the Universe".

(5) Ling means Mark and the Mark of male is Phallus; thus Ling may mean phallus; that is the sign of Shiv. The question comes up why Ling is the way it looks. If there is smoke, there is a fire. The rising smoke is the sign that fire is present in an unseen distant hill. When you see Washington Monument, you think of Washington, though the monument does not look anything like President Washington. Thus just like Washington Monument is an abstract form of President Washington, Ling is an abstract form of Shiv and a generative principle. Shiv, according to Shaivites, is the First God; he is the Father of all beings; he is the First One to say, "Aham" meaning I, the First I in the Universe. There was no one else in the Universe except his Shakti (Power by His side).

Being (Sattaa) or the Supreme Being is of two kinds: A-Roop (Formless) and Sa-Roop (with form). A-Roop kind consists of Aham and Idam ( I and This). Aham is Prakaash and Idam is Vimarsh. Shiv and the heart of the Lord are Shiv-Shakti. Shiv is Chitta (Consciousness) and Shakti is Chid-Roopinee (Form of Consciousness). Chid-Roopinee = Chaitanya Roop. He is Par and she Paraa. The long "A" (Paraa) indicates female gender. The short a is male (Par) He is Shav (dead) and She is Shakti. If Shiv does not have Shakti (power), he is dead. He is Akul and she is Shakti. He is Shiv and Shakti is Shivaa or Shive. Thus, Shiv and Shakti is a unitary force from which the First I (through Shakti) became He, She, and It, which cover everything in the Universe; I or One became many. Shiv experiences the Universe as Paraa-Shakti (Paranad and Paravak). Idam exists as Paraa-Shakti. Lingam is an obelisk well grounded in a labial vestibular pedestal and the aniconic form (Nish = Kalaa) of Shiv. The three white stripes, sandalwood, red dots, and the serpent at the base of the lingam have significance. The serpent represents Shakti-Kundalinee.

A-Ling is one without mark - The Great Supreme Vyakt (manifestation) is beyond the ancient Aatm Mahaan, which is beyond intelligence, the essence of the mind; the latter is beyond the mind, which is beyond the senses. Beyond A-Vyakt (Unmanifest entity) is the entity who is all-pervasive and devoid of any mark (A-Ling). Knowing him brings liberation and immortality. Ling consists of Praakritik (natural) elements such as Buddhi, Ahankaar, Manas, Indriyaan and Tanmaatraa (intellect, ego, mind, sense organs and subtle elements); the Supreme is not tainted with these factors or limitations; therefore, it is not subject to Sansaar (life on earth). Ling status marks manifest Eeshwar, while A-Ling status carries no marks and is unknowable; it is A-Vyakt, primordial, undifferentiated, unmanifest, noumenal state. Eyes have never seen this form, and they can never see it also as it has no form. He can be known only by (spiritual) heart, mind and (spiritual) wisdom. Mental focus helps apprehend Reality. When the five senses (vision, hearing, taste and speech, touch and grasp) and the mind come to a standstill and the intellect does not stir, that is the highest state (Param). When the Prakriti-bound senses and the mind come to an arrest, the spiritual world that lies beyond the grasp of the senses comes into access. This is Yog; it needs control of the senses; distractions make Yog come and go.

Other Interpretations of Shiv Ling
Ling, besides as the symbol and form of Shiv, religious scholars have given various interpretations of Shiv Ling. Here is the brief description of some of the popular theories and interpretations related to Shiv Ling and its origin:

Worship of the Phallus
According to some scholars, worship of Shiv Ling in effect means worship of the reproduction function. For, they say that the other meaning of the Sanskrit word "Ling" is gender in general and phallus (the male reproductive organ) in particular (see above No 1 and 2). They believe that the base of the Lingam corresponds to the Yoni which mean vagina or the female reproductive organ. Correspondence of Ling and Yoni in a Shiv Ling is therefore interpreted as the representation of the process of copulation for creation. Scholars further opine that the Kalash (container of water) that is suspended over the Shiv Ling from which water drips over the Ling continuously also correspond to the idea of intercourse. Connecting the origin of Shiv Ling to the early Indus Valley civilization, scholars opine that tribes of the Indus Valley took to the togetherness of Lingam and Yoni in a Shiv Ling as the point of energy, creation and enlightenment.

Interpretation in Tantra
According to Tantra, Lingam is a symbol of Shiv's phallus in spiritual form. They say, the Ling contains the soul-seed within which lies the essence of the entire cosmos. The Ling arises out of the base (Yoni) which represents Paarvatee according to some; or Vishnu, Brahmaa in female and neuter form according to others.

Interpretation in Puraan
Puraan, especially the Vaaman Puraan, Shiv Puraan, Ling Puraan, Skand Puraan, Matsya Puraan and Vishwa-Saar Prakaash attribute the origin of Shiv Ling to the curse of sages leading to the separation of and installation of the phallus of Lord Shiv on earth. Some also refer to the endlessness of the lingam to be linked to the egos of Vishnu and Brahma (read this story "Greater" here).

Interpretation of Shiv Ling as an Abstract Symbol of God
Some scholars of the Hindu scriptures say that Ling is merely an abstract symbol of the God. They point towards several legends in Hinduism where a sundry rock or even a pile of sand has been used by as a Ling or the symbol of Shiv. Citing a particular instance they say, Arjun once fashioned a Ling of clay when worshipping Shiv. Scholars of Puraan, thus argue that too much should not be made of the usual shape of the Ling. Scholars say that the interpretation of Shiv Ling as an abstract form of God is also consonant with
philosophies that hold that God may be conceptualized and worshipped in any convenient form. The form itself is irrelevant, as the divine power that it represents is all that matters. Scholars thus say that Shiv Ling represent the formless Nir-Gun Brahm or the formless Supreme Being.

And the last main thing .... By the science it is proved very clearly, a Shiv Ling is a huge power energy storage shape (more than that there is one more Vigraha which can store energy, that is Sarp ranji Vigraha which is seen in our Sarp kaadukal), that is why in this world total around 207Atomic reactors are there, and all of them are the same shape of Shiv Ling. All reactors need continuous flow of heavy water circulation to keep them in cooling condition, same like the Dharakadaram in Shiv Temple over Shiv Ling.

Swaayambhoo Ling
Swaayambhoo Ling is that Ling which is not man made and has come out itself from the Earth. Such Ling are considered very auspicious and do not need any Sthaapanaa (establishment rituals). They can be worshipped as such.

Images of Ling
The images of Shiv Ling are usually made of stone, carved or naturally made, but may be made of metal, precious gems, crystal, earth or transitory matter like water/ice; but according to Kaaran Agam, this image can be of 12 materials - sand, rice, cooked food, river clay, cow dung, butter, Rudraaksh beads, ashes, sandalwood, Darbhaa grass, a flower garland, or molasses. Arjun fashioned a Ling of clay when worshipping Shiv; even Shree Raam also established a clay Ling at Raameshwaram while going to Lankaa; Raavan while going to fight with Sahastraarjun, came to Narmadaa River and liked the place there so he established a Shiv Ling there and started worshipping it. This shows that there is no need to pay too much attention to the material to make it (see the list of so many materials used by Devtaa to make Shiv's Ling). It also shows that God has just to be conceptualized and worshipped in any convenient form, at any place, at any time - the form itself is irrelevant, it matters only the Divine power it carries. Originally 12 principal Ling (Jyotirling) existed in India, of which the most popularly known are (1) Somnaath in Gujaraat, (2) Mahaakaal at Ujjayinee, (3) Vishweshwar or Vishwanaath at Banaaras, (4) Raameshwaram in Tamil Naadu, (5) Kedaarnaath Temple in Uttaraakhand, etc.

Forms of Shiv Ling (Panch Bhoot Ling)
There are five forms Ling - (1) Prithvi, (2) Vaayu, (3) Agni, (4) Jal, and (5) Aakaash.
(1) Earth Ling - Prithvi - means made of earth. This Lingam is in Raameshwarm, Tamilnaadu, established by Shree Raam while going to Lankaa. (Some say that it is in Kaacheepuram, Tamilnaadu)
(2) Vaayu Lingam - means made of air. It is in Kaalaahastee, Aandhra Pradesh.
(3) Tejo Ling - means made of Agni (fire) is in Arunaachal Temple in Tiruvannaamalaai, 80 miles Southwest to Chennai. It is one of the largest temples in the world extending its boundaries over 25 acres and containing a courtyard with a 1,000 pillars. The Red Mountain of Tiruvannaamalaai is the Fire Lingam. Empty space is dark matter and thus is Shiv.
(4) Jal Ling - means made of water. This Ling is naturally made at Amarnaath, in Kashmeer, every winter,  by dripped water from the ceiling of the cave and dropping on the floor, eventually taking a form of Shiv Ling. Hundreds of thousands of people go for pilgrimage there. (some say it is Jambukeshwar, Trichy, Tamilnaadu)
(5) Aakaash Ling - means made of space. It is in Chidaambaram, Tamilnaadu. When the curtain to Chitta Sabah (Hall of Consciousness) in Chidaambaram is drawn, what you see is an empty space representing Sky Lingam; this is the Chidaambaram Rahasya (the Secret of Chidambaram); the Lord exists in all the apparently empty spaces from the atom to the vast spaces of cosmos; thus, he is the Cosmic Dancer.
-------------------------------------------------------------

Asht Moorti
Ved speak of Asht Moorti of Lord Shiv - (1) Sarv, (2) Bhav, (3) Rudra (4) Ugra, (5) Bheem, (6) Pashupati, (7) Mahaadev, and (8) Eeshaan. Puraan interpret them thus - (1) Sarv for earth, (2) Bhav for water, (3) Rudra for fire, (4) Ugra for wind, (5) Bheem for space, (6) Pashupati for Yajamaan, (7) Mahaadev for Chandramaa - Moon, and (8) Eeshaan for Soorya - the Sun. The following are the Shiv temples worshipped in their primal form only without any special idols representing them.

(1) Sarv - Bhoomi Ling in Kaancheepuram, in Tamilnaadu. It is in Shiv Kaanchee Kshetra, where he is in the form of Kshiti Ling in the Ekaamra tree (Aamra means Mango in Sankrit), which yields only one fruit per year. Paarvatee Jee worshipped this form first. There is no Abhishek done with water at this shrine, only the jasmine oil is used instead. The Devee’s name here is Kamaakshee. All the desires of the devotees are fulfilled with her gracious eyes.

(2) Bhav - Jal Ling in Tiruvanaikoil, (Jambookeshwaram) in Tamilnaadu. This temple is located on the outskirts of Trichee, where Lord Jambookeshwar is seated and showers all his blessings to his devotees. The devotees can see it from the outside of Garbh Grah the water bubbles coming out from Panipeetham. There is a Jamboo tree here which is very old and very big. The legends say Lord Shiv wanted to stay here along with the Jamboo tree. So the devotees treat this tree as sacred as the Lord.

(3) Rudra - Agni or Tejo (Divine Light) Ling in Tiruvannamalai, in Tamilnaadu – Arunaachaleshwara. In Tiruvannamalai, Lord Shiva is seated in the form of Tejoling. The whole mountain appears to be a Ling. As a result of Paarvatee’s great penance, a sharp spark of fire came from Arunaachal and took shape as Arunaling.

(4) Ugra - Vaayu Linga in Kaalaahastee, in Aandhra Pradesh. This Shree Kaalaahasteeshwar temple is situated on the banks of Swarn Mukhee River in Shree Kaalaahastee. Spiritually elevated souls only can see that there is a strong wind blowing around the Ling. Bhakt Kannappaa story is connected to this temple. Even animals got salvation by worshipping this Lord. Three animals – Cobweb (Shree), Kaal (snake), and Hastee (elephant) prayed to God with utmost faith and devotion and attained Moksh. One can see their symbols on the Shiv Ling even today.

(5) Bheem - Aakash Ling in Chidaambaram, in Tamilnaadu. This Kshetra is on the banks of Kaaveree. There is no  Moorti (Shiv Ling) in the temple of Garbh Grah, so no one can see the Lord’s Moorti, except the very high spiritual souls. There is a space in the Garbh Grah and many Aabharan (ornaments) are decorated there and the devotees assume the God is seated there. A very beautiful Nataraaj Moorti is situated in outer Garbh Grah for worship and for the satisfaction of the devotees.

(6) Pashupati - Yajamaan (Lord) Ling in Kaathmaandu in Nepaal. In Nepal, Pasupatinaath Kshetra is famous and the Lord here is in human form. The devotee can see the God up to his waist only. The Moorti is decorated with Gold Kavach. Nobody can enter into the Garbh Grah except the Archak (priest), not even the King of Nepal. This temple is also wish fulfilling.

(7) Mahaadev - Chandra Ling in West Bengal. Chandranaath Ling is situated in West Bangaal, 34 miles away from Chatagaanv City. Many sacred Teerth surround this Kshetra. Devee Puraan lauds this Kshetra greatly.

(8) Eashaan - Soorya Ling in Konaark Temple in Udeessa, near Jagannaath Puree Kshetra. Konaark is now in ruins and the temple is in fragments and now, devotees cannot see any God or Goddess here. The legend says that Shree Krishn’s son Saamb once suffered from leprosy and was cured by worshipping the Sun God and the Shiv Ling here and since then this Kshetra became a remedy center for all diseases. Even in these days the worship is going on with same faith and devotion.

The Bijleshwar Mahaadev Ling (incidence of Vashishth Jee in Rig Ved), in Kulloo in Himaachal Pradesh. Every 12 years a metal staff, about 60 feet high, absorbs the lightening and breaks into pieces. Later the pieces of the Ling are covered with Sattoo and butter and thus restored ... of course till another lightening breaks it. This is done in a secrecy by the temple priests etc.

Shiv Ling (6,543 m high) is also a mountain in Uttaraakhand (the Garhwal region of Himaalaya). It arises as a sheer pyramid above the Gangotree Glacier. The mountain resembles a Shiv Ling when viewed from certain angles, especially when traveling or trekking from Gangotree to Gomukh.

The Structure of Shiv Ling
Shiv Ling consists of three parts - (1) the bottom part, a four-sided, remains under ground and represents Brahm, (2) The middle part which is eight-sided remains on a pedestal and represents Vishnu, (3) and the top part which is actually worshipped is round pillar with a rounded top is Shiv. The height of the round part is normally 1/3rd of its circumference.