Deltoid: An adjective and a noun.

Adjective: In the shape of delta, the fourth letter in the Greek alphabet. A capital delta is a triangle. Hence, deltoid means triangular in configuration.

Noun: The large muscle, roughly triangular in shape, that stretches from the clavicle (collarbone) to the humerus (the long bone in the upper arm) and so covers the shoulder. When the deltoid is contracted (flexed), it moves the arm away from the side of the body.