Ace

The ace is the best pitcher on a team's staff. In the earliest days of the game it was restricted to starting pitchers, but today a team might have both an ace starter and an ace reliever (usually the closer).

Some people claim that the term was first used in reference to Asa Brainard, the star pitcher for the 1869 Cincinnati Red Stockings. A more likely explanation is that it derives from card playing, where the ace is the highest ranking card in a suit. A number of pitchers have been nicknamed "Ace" whether or not they were actually the best pitcher on the staff.