The muscle that produces the movement in an exercise, also called the prime mover, such as the biceps in a curl.
Every movement has one. The quadriceps are the agonist in a leg extension, the pecs in a bench press, the glutes in a hip thrust. Naming it is how you decide whether an exercise belongs in a program: if the agonist is not the muscle you are trying to train, the exercise is filler.
Agonist is a role, not a label attached to a muscle for life. The hamstrings are the agonist in a leg curl and an antagonist in a leg extension, in the same session, an hour apart.
Prime mover means the same thing in practice. Some textbooks use prime mover for the single biggest contributor and agonist for the group, but no trainer programs differently because of the distinction.