A term in Tibetan Buddhism used to describe someone who has consciously chosen the exact circumstances of his rebirth, as opposed to someone who is simply reborn according to his karma and the state of mind at the time of his death.
Generally, tulkus choose their next rebirth for a specific purpose, such as to continue the work they started in the previous life time.
For example, the Dalai Lama is believed to be the tulku of the previous Dalai Lama.