Medical Definition of teres major
: a thick somewhat flattened muscle that arises chiefly from the lower third of the axillary border of the scapula, passes in front of the long head of the triceps to insert on the medial border of the bicipital groove of the humerus, and functions in opposition to the muscles comprising the rotator cuff by extending the arm when it is in the flexed position and by rotating it medially
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