opponent-n; 3 Senses

Sense Number 1: a contestant, one who competes to win

Commentary: OPPONENT[+entity][+person][+contestant]
NOTE: implies a game with a winner and loser

After winning the first round, we faced a much tougher opponent in the second round.
A computer proved to be a worthy opponent to the world chess champion.
The congressman beat his opponents by a landslide.

WordNet 3.0 Sense Numbers: 1

Sense Number 2: one who disagrees, objects to

Commentary: OPPONENT[+entity][+person][+disputant]
NOTE: does not imply a game or competition, just a general disagreement

He is an outspoken opponent of socialist economic policies.
There are few opponents to this bill in the House.
Bob seemed to be an opponent to every one of Mary's proposals to the committee.

WordNet 3.0 Sense Numbers: 2

Sense Number 3: a muscle that counteracts another muscle

Commentary: OPPONENT[+entity][+structure][+anatomical][+muscle][+counteracting]

Which muscle is the opponent of the triceps?

WordNet 0.0 Sense Numbers: 2