Final answer:
The deltoid is the triangular muscle that abducts the arm and covers the shoulder, responsible for various shoulder movements.
Step-by-step explanation:
The triangular muscle that abducts the arm and covers the shoulder is the deltoid. This muscle is named for its shape, resembling the Greek letter delta, which is triangular. The deltoid is a significant muscle in the shoulder region and is responsible for a range of movements including abduction, flexion, medial rotation, extension, and lateral rotation of the arm. The deltoid muscle is the major abductor of the arm but also assists in several other motions, giving the shoulder its versatility in movement.