catur yoni (Sanskrit). The four ‘wombs’ or modes of birth. According to traditional Buddhist teachings, life arises in one of these four ways: from an egg, from a womb, from heat or exudation (thought to be the case for insects), and ‘spontaneous generation’ (as in the case of devas who generate a body through magical power).

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