One who makes an apology; one who speaks or writes in defenseof a faith, a cause, or an institution; especially, one who argues indefense of Christianity.
One of those annoying douches who apologizes before saying something horribly offensive, ignorant, or demeaning to you. The apology is supposed to make it nicer somehow--or they're attempting to be PC (which is annoying as well).
These type of people also lose credibility on their beliefs since it conveys less confidence about their beliefs.
An apologist is someone who argues in defense of a controversial truth, usually but not always associated with Christianity.