It's an auxiliary muscle. For example, if you are doing squats, your hamstrings are an auxiliary muscle group; they work together with your quads and buttocks, which are the primary muscles being worked.
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Synergists muscles in a bench press are triceps and anterior deltoids. When performing a bench press if an athlete wants to make sure that most of the load will be distributed on his/hers pectoralis major (chest muscles) make sure to contract those muscles before the actual push...otherwise synergists will do big part of the work.
A synergist muscle helps a prime mover muscle in creating a specific movement. It assists the prime mover by reducing unnecessary movements and helping stabilize the joint. Synergist muscles work together to produce coordinated movement.
The synergist to the smiling muscle (zygomaticus major) is the orbicularis oculi muscle, which is responsible for the contraction around the eyes and contributes to the overall expression of a smile.
A synergist muscle is a muscle that aids another by promoting the same movement.
The muscle that aids the jaw in closing is the temporalis muscle. The masseter is the muscle that primarily moves the jaw.
The peroneus longus and peroneus brevis muscles are synergists for eversion of the foot. They work together to assist in turning the foot outward.
Piriformis
The rectus femoris muscle is a synergist of the iliopsoas. It works together with the iliopsoas to flex the hip joint.
infraspinatus muscle
agonist, antagonist, synergist
Gastrocnemius
Latissimus dorsi is the synergist of the teres major.Specifically, a synergist is a muscle which acts with another. The latissimus dorsi helps adduct and rotate the shoulder to elbow bone called the humerus. The teres major is a shoulder-related muscle.
brachialis