1 A person or animal engaged in a struggle. Compare antagonist (1).
2 A muscle that contracts in the same direction as another. Compare antagonist (2).
3 A substance that binds to neuroreceptors and has the same effects in exciting or inhibiting the postsynaptic cells as the neurotransmitter that would otherwise bind to those sites. See also synergist. Compare antagonist (3). [From Greek agon a festival of competitions, from agein to lead]