what is the deltoid
deltoid. 1. triangular. 2. the deltoid muscle. deltoid muscle. a lateral muscle of the shoulder, between the scapula and the humerus that helps flex the shoulder and abducts the arm. See also Table 13.. triangular. 2. the deltoid muscle. deltoid muscle. a lateral muscle of the shoulder, between the scapula and the humerus that helps flex the shoulder and abducts the arm. See also Table 13.