A muscle that assists the prime mover in a movement, either by adding force or by holding a joint steady so the prime mover can work.
In a bench press the pecs are the agonist and the front delts and triceps are synergists. In a pull-up the lats are the agonist and the biceps, rear delts and rhomboids are synergists.
Synergists are usually smaller and usually fail first, which is what makes them a programming problem. A client whose bench stalls often has triceps that quit before the chest does, and the answer is direct triceps work, not more benching. The test is simple: if a close-grip bench is far weaker than it should be relative to their competition grip, the triceps are the limit.
Stabilisers are the other kind of synergist. The rotator cuff adds almost no force to an overhead press but holds the shoulder centred while the delts push, which is why cuff work belongs before pressing volume goes up, not after.