There are several legends as to how Ganesha broke one of his tusks, giving him the name, Ekadanta, the one with a single tusk.
The first story is recounted in some excerpt from the Upodghata Pada of the Brahmanda Purana. This story pertains to his battle with Parashurama, who was one of the incarnations of Vishnu. Born on earth as a normal human & a great devotee of Lord Shiva, he meditated on Shiva and obtained the divine axe, Parashu, with whose help he waged wars against all the erring princes and cleared the world of their evil. After successfully defeating his enemy Kartavirya Arjuna and the kings allied with him, he wanted to thank Shiva for giving him the power to fight these enemies. Parashurama went to Mount Kailash to pay his respect to Shiva, but Ganesha who was guarding the entrance to his father’s chambers stopped him. Parashurama was enraged that he was being prevented from seeing Shiva, and so he hurled his axe at Ganesha. Ganesha realized that this was the axe presented to the sage by his father. Dodging it would be an act of disrespect to the axe, so Ganesha stood his ground, and the axe fell on one of his tusks, breaking it. Hearing the commotion, Shiva & Parvati appeared before them and chastised Parashurama, who then worshipped Ganesh and obtained his forgiveness and blessings.
Another popular story as to how Ganesha lost his tusk is as follows. Veda Vyasa decided to compose the huge epic Mahabharata. He needed somebody to write down his composition. Ganesha, being the God of knowledge, was requested to do this job. Ganesha agreed on one condition. Veda Vyasa should dictate all he wants to write at one stretch and without break. While agreeing to Ganesh’s condition, Ved Vyasa put forth another condition of his own. He told Ganesha that the latter would have to continuously and without pause transcribe the contents of the epic. An agreement was reached basis this understanding and soon Ved Vyasa and Ganesh began work. However, as they were half-way through, Ganesh realized that his pen had worn off and he did not have a replacement. At this point, remembering the conditions put forward by the sage, Ganesha immediately broke one of his tusks and dipping it in ink, used it to transcribe the rest of the Mahabharat as narrated to him by the great sage.
There are some more versions as to how our favorite Ganpati got to have just one tusk. There are also symbolic implications of the single tusk that the wise person is beyond duality, of seeing all as one and ourselves an integral part of the whole!
We will be back with some more interesting tales of the Vighnaharta soon!