catur-nimitta (Sanskrit). The ‘four signs’ seen by Prince Siddhartha (see Siddhartha Gautama) which caused him to renounce the world and take up the religious life. The four signs were (1) an old man, (2) a sick man, (3) a corpse, and (4) a religious mendicant. It was predicted at the Buddha's birth that if he saw these four signs he would abandon worldly life, and his father Śuddhodana therefore tried to shield his son from these unpleasant experiences by distracting him with luxuries inside the palace, but his efforts were unsuccessful. Once confronted with the realities of life and the problem of human suffering (duḥkha), Siddhartha left the palace and became a religious mendicant (śramaṇa).

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