Lord Shiva, also known as Mahadev, is one of the supreme gods in the Trinity of Brahma-Vishnu-Mahesh. Unlike other gods, whose temples house their idols for worship, Shiva temple houses his representation – Shiva Linga. The word linga in Sanskrit means the mark, symbol or emblem; hence Shiva Linga literally means the symbol of Shiva.
The oldest mention of anything similar to Shiva Linga is found in Arthrvaveda. A hymn in the Veda praises a yupa sthambh – a sacrificial post, which seems to have no beginning or an end – resembling the eternal universe. Once the yagna (sacrifice) was over, its ashes, smoke along with the ox that carried the wood for sacrificial fire flames transformed into the brightness of Shiva’s body riding on the bull. Hence the Yupa sthambh eventually has been replaced by Shiva-Linga.
There is also another story mentioned in Shiva Purana. Once Lord Brahma and Lord Vishnu got into an argument over who is superior. Both gods fought in vain trying to prove their supremacy over other. When the intensity of the battle grew, other gods approached Lord Shiva asking him to arbitrate. Lord Shiva assumed the form of a blazing fire pillar which appeared in front of them. He challenged both Lord Brahma and Lord Vishnu to find the beginning and an end of the pillar. Arbitrarily both Lord Brahma and Lord Vishnu decided to find each end. Hence Lord Brahma went to skies looking for a beginning and Lord Vishnu went to the underworld trying to look for an end but both were disappointed not to find it. Exhausted in his journey, Lord Brahma met Ketaki (a flower) and made her his ally in the false claim that he had indeed found a beginning. When he returned and declared this false claim, the fire pillar split open and Lord Shiva stepped out in his magnificence. Humbled with this vision both Lord Brahma and Lord Vishnu accepted Lord Shiva’s supremacy. Angry at Lord Brahma for lying, he cursed him that no one will worship him, hence there are barely any temples dedicated to Lord Brahma. He cursed Ketaki too, rendering her unfit to be used in puja. The eternal fire pillar is symbolised with the conical oval shape of Shiva Linga as a formless universe bearer which radiates with all the celestial energy and provides focus for meditation.
It is believed that Shiva manifested himself in the form of Shiva Linga on the fourteenth day of Krishna paksha (dark side of the moon) in the Phalgun month as per Hindu calendar. This auspicious day is also celebrated as Mahashivratri by Hindus. Devotees offer prayers along with bel-patra and abhishek of milk to Shiva-Linga for happiness and prosperity.