Original: A tulku (, also tülku, trulku) is an enlightened Tibetan Buddhist lama who has, through phowa and siddhi, consciously determined to take birth, often many times, in order to continue his or her Bodhisattva vow.
Simplified: A tulku (also tülku, trulku) is a Tibetan Buddhist lama who has achieved the ability to consciously determine to be reborn, often many times, in order to continue their Bodhisattva vow.