Antagonist (noun)

1. A person or group who is opposed to, competes with, or tries to harm or defeat another.
2. A muscle that acts in opposition to another, especially one that relaxes while the other contracts.

Origin:

Late 16th century: via latin from greek antagōnistēs, from ant- 'against' + agōnistēs 'competitor'.

Examples:

1. The hero of the story is fighting against the antagonist, who is trying to take over the kingdom.
2. In a tennis match, the two players are antagonists competing against each other.
3. The antagonist in the play was a deceitful and manipulative character.
4. The antagonist muscle helps to control movement and maintain balance.
5. In a political context, the two major parties may be seen as antagonists with opposing ideologies.