Definition: a person who champions the less accepted cause for the sake of argument
Devil's advocate has gone through a number of shifts in meaning over the past several hundred years. The earliest meaning was a somewhat literal one: a person thought wicked enough to be said to be advocating on behalf of the devil.
If therefore our companions delight in sin, let not vs delight in them, but flee their societie as being the diuels aduocates to solicite vs vnto wickednesse; and let vs take heede lest whilest wee labour to maintaine friendship with men, we doe not proclaime enmitie against God.
—John Downame, Foure Treatises, 1609
And therefore let vs stop our cares against these worldly allurements, whereby wicked men intice vs vnto sinne, and like the diuels aduocats perswade vs to by the hazarding of our soules, to obtaine worldly vanities.
—John Downame, The Second Part of the Christian Warfare, 1611
In the 18th century a new sense was added, peculiar to Roman Catholicism: "an official of the Congregation of Rites whose duty is to point out defects in the evidence upon which a demand for beatification or canonization rests or in the character of the person for whom the honor is sought."
In modern use the term is most commonly applied to a person who argues an unpopular or less accepted position.