The three-headed muscle at the back of the upper arm, worked by push-downs and dips, is commonly called the triceps. Its full anatomical name is the triceps brachii, meaning “three-headed muscle of the arm.”
The triceps has three heads: the long head, lateral head, and medial head. All three contribute to extending the forearm at the elbow, which is why exercises that involve pressing or straightening the arm recruit the muscle. The long head also crosses the shoulder joint, giving it an additional role in shoulder movement.
The triceps acts as the main antagonist to the biceps brachii. When the triceps contracts to straighten the elbow, the biceps generally relaxes or lengthens. Push-downs, dips, close-grip presses, and overhead extensions can all train the triceps, though the exact emphasis varies with body position and technique.
A common mix-up is assuming that “triceps” refers to a single undivided muscle. In everyday English it is the standard name for the muscle group, and the same word is commonly used whether discussing one arm or both arms. The lower leg also contains a separate group called the triceps surae.