One who has taken refuge in the Buddha, the Dharma, and the Sangha.
Not a follower of a set of teachings, but one who actively searches for the truth, for seeing things as they are rather than as they appear to be, and then realizes his understanding is similar to that of Gautama Buddha.
In traditionally Buddhist countries, also one who has been raised according to the tenets of Buddhism, and has accepted them as his own, especially if he has taken the Three Refuges.