Masseter

n. short muscle which assists in closing the jaws by raising the mandible or lower jaw (Anatomy)

In human anatomy, the masseter is one of the muscles of mastication. It is particularly powerful in herbivores to assist when they are chewing plants.
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Masseter /mas·se·ter/ masseter muscle. masseter´ic mas·se·ter n. A muscle with origin from the inferior border and medial surface of the zygomatic [more]Masseter - Community and Resources

Noun
1. a large muscle that raises the lower jaw and is used in chewing
(hypernym) facial muscle

(n.)
The large muscle which raises the under jaw, and assists in mastication.