Ace

Concise Oxford English Dictionary © 2008 Oxford University Press:
ace/eɪs/

▶noun

• 1 a playing card with a single spot on it, the highest card in its suit in most games.

• 2 informal a person who excels at a particular activity.

• 3 Tennis a service that an opponent is unable to return.

• 4 Golf a hole in one.

▶adjective informal very good. ▶verb informal

• 1 (in tennis) serve an ace against.

• 2 Golf score an ace on (a hole).

• 3 N. Amer. achieve high marks in (a test or exam).

– phrases
ace up one's sleeve (or N. Amer. in the hole) something effective kept in reserve.
hold all the aces have all the advantages.
within an ace of very close to.

– origin ME (denoting the ‘one’ on dice): via OFr. from L. as ‘unity, a unit’.