ace 
(ās)
(ās)n.
1.
a. A single spot or pip on a playing card, die, or domino.
b. A playing card, die, or domino having one spot or pip.
2. In racket games:
a. A serve that one's opponent fails to hit.
b. A point scored by such a serve.
3. The act of hitting a golf ball in the hole with one's first shot.
4. A military aircraft pilot who has destroyed five or more enemy aircraft.
5. An expert in a given field.
adj.
Top-notch; first-rate.
tr.v. aced, ac·ing, ac·es
Idioms: 1. To serve an ace against in racket games.
2. To hit an ace on (a hole) in golf.
3. Slang To get the better of (someone): a candidate who aced his opponents in the primaries.
4. Slang
a. To receive a grade of A on: She aced the exam.
b. To perform with distinction on: aced the interview.
ace in the hole/up one's sleeve
A hidden advantage or resource kept in reserve until needed.
within an ace of
On the verge of; very near to: came within an ace of losing the election.
[Middle English as, from Old French, from Latin, unit.]