The word Buddha is a Sanskrit word which means "The Awakened One", one who is awakened to Reality, who understands true nature of the mind, the world, and all sentient beings.
Enlightenment is a state of perfect knowledge or wisdom, combined with infinite compassion. Knowledge in this case does not mean merely the accumulation of data or a description of the world of phenomena down to the finest details. Enlightenment is an understanding of both the relative mode of existence (the way in which things appear to us) and the ultimate mode of existence (the true nature of these same appearances). This includes our own minds as well as the external world. Such knowledge is the basic antidote to ignorance and suffering.