opponent
A person who competes or fights against you in something.
An opponent is someone who competes against you in a game, sport, debate, or contest. In chess, your opponent sits across the board trying to checkmate your king while you try to checkmate theirs. In soccer, the opposing team's players are your opponents. During a spelling bee, the other students competing for first place are your opponents.
In a court case, lawyers can be opponents arguing different sides. In an election, candidates are opponents seeking the same office.
Being opponents doesn't mean being enemies. Two tennis players can be fierce opponents during a match, then friends afterward who respect each other's skills. Good opponents actually make you better by pushing you to improve and try harder. A strong opponent reveals weaknesses you didn't know you had and forces you to develop new strategies.
People sometimes use opponent more broadly for anyone who opposes an idea or plan. Someone who speaks against building a new playground might be called an opponent of the proposal, even though they're not competing in a traditional sense.