Deltoid Health Dictionary

The triangular muscle of the shoulder region that forms the rounded flesh of the outer part of the upper arm, and passes up and over the shoulder joint. The wide end of the muscle is attached to the shoulderblade and the collarbone. The muscle fibres meet to form the apex of the triangle, which is attached to the humerus (upper-arm bone) halfway down its length. The central, strongest part of the muscle raises the arm sideways. The front and back parts of the muscle twist the arm.

The powerful triangular muscle attached above to the collar-bone and shoulder-blade, and below, by its point, to the humerus, nearly halfway down the outer side of the upper arm. Its action is to raise the arm from the side, and it covers and gives roundness to the shoulder. (See also MUSCLE.)

n. a thick triangular muscle that covers the shoulder joint (see illustration). It is responsible for raising the arm away from the side of the body.