Terminology
It is sometimes called a three-headed muscle because there are three bundles of muscles, each of different origins, joining together at the elbow. Though a similarly named muscle, the triceps surae, is found on the lower leg, the triceps brachii is commonly called the triceps.
Historically, the plural form of triceps was tricipites, a form not in general use today; instead, triceps is both singular and plural (i.e., when referring to both arms). The triceps also make up approximately 2/3 of the muscle mass in the arm.
Read more about this topic: Triceps Brachii Muscle