Triceps

A triceps is a muscle with three heads. The triceps brachii, on the back of the upper arm, extends the elbow.

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Triceps brachii: The three-headed muscle on the back of the upper arm that straightens the elbow. This is the specific muscle most commonly meant by “triceps.”
Skeletal muscle: Muscle tissue attached to the skeleton that produces voluntary movement and helps maintain posture. The triceps is a skeletal muscle controlled primarily by voluntary motor commands.
Triceps extension: A resistance exercise that straightens the elbow against an external load. It directly trains the triceps through its principal joint action.
Biceps brachii: A two-headed muscle at the front of the upper arm that flexes the elbow and supinates the forearm. It chiefly bends the elbow, opposing the triceps’ extension action.
Elbow extension: The movement that straightens the elbow by increasing the angle between the upper arm and forearm. The triceps brachii is the principal muscle producing this movement.
Muscle head: A distinct proximal portion of a muscle with its own origin. The name “triceps” describes a muscle organized into three such portions.
Close-grip bench press: A bench-press variation using a narrower hand position that increases the triceps’ contribution. It loads the triceps during both elbow extension and pressing.
Brachialis: A muscle deep to the biceps brachii that is a powerful elbow flexor. It flexes the elbow rather than extending it.
Long head of triceps brachii: The triceps brachii head that originates from the scapula and crosses the shoulder joint. Its shoulder-crossing origin gives it actions beyond elbow extension.
Olecranon: The prominent proximal projection of the ulna that forms the tip of the elbow. The triceps tendon attaches here to extend the elbow.