An opponent is your competition, whether it's a rival hockey team, or the other finalist in the chess tournament. Coaches tell you to never underestimate your opponent.
Someone who competes or fights against you is your opponent. If you're arguing that chocolate ice cream is better than vanilla, the vanilla ice cream fan is your opponent. An opponent can also be someone who opposes, like an opponent of gun control laws. The Latin word opponent means setting against, and it was first used in English to describe the person who would begin a certain type of philosophical debate.