Brewer's: An Advocate
One called to assist clients in a court of law. (Latin, advocare.)
The Devil's Advocate
One who brings forward malicious accusations. When any name is proposed for canonisation in the Roman Catholic Church, two advocates are appointed, one to oppose the motion and one to defend it. The former, called Advocatus Diaboli (the Devil's Advocate), advances all he can against the person in question, the latter, called Advocatus Dei (God's Advocate), says all he can in support of the proposal.