Great Renunciation 

Name given to the important event in the Buddha's life at the age of 29 when he left his father's palace and abandoned the world to become a homeless mendicant. A popular theme in art and literature, the Buddha is depicted riding out on his white horse, Kanthaka, accompanied by his charioteer, Channa.

A Dictionary of Buddhism, Oxford University Press, 2003, 2004 (which is available in electronic version from answer.com)