tülku  (Tibetan, sprul-sku). Literally, ‘Emanation Body’, the term is descriptive of certain teachers in Tibet who are thought to reincarnate over a number of generations. Though known in principle in India, the tülku system was first formally developed and adopted in Tibet by members of the Karma-kagyü lineage and then spread to the other schools of Buddhism.

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