The triceps is the big muscle at the back of your upper arm. Its job is to straighten the elbow, which makes it the partner and opposite of the biceps at the front, which bends the elbow. That pairing is the easiest way to remember it: biceps pulls the arm closed, triceps pushes it open — so push-ups, pushing a heavy door, throwing a ball. It matters for two reasons in real life. In the gym, people talk about training triceps because it is actually the larger of the two muscles, so it does more than the biceps to make an arm look big — a fact that surprises students who only ever curl. In medicine and biology you meet 'triceps' in careful anatomical names. The spelling is the trap: the word ends in -s even when you mean one muscle, so 'a triceps' is correct, and the plural is 'triceps' too (a few writers use 'tricepses', but it looks odd). Gym speakers everywhere shorten it to 'tris' — chest and tris today.