Final answer:
The deltoid is a large, triangular shoulder muscle responsible for moving the arm in various directions, including abduction and rotation.
Step-by-step explanation:
The deltoid muscle is a large, triangular-shaped muscle located on the shoulder. It is crucial for the movement of the arm, as it is involved in abduction (movement away from the midline of the body), flexion, and medial rotation, as well as extension and lateral rotation. The name 'deltoid' is derived from the Greek letter delta which resembles a triangle. This muscle's multipennate structure allows different portions of it to be stimulated separately, which enables the variation in the direction of arm movements.