A press-up is an exercise: you lie facing the ground, hands on the floor, and push your body up with your arms, then lower it again. In Britain people say press-up; in the United States the same movement is a push-up. Gym teachers, soldiers, and sports coaches use it all the time. You do press-ups; a trainer may tell you to drop and give them twenty. It is informal, physical language — not a fancy word. Though actually, people stretch it a little: knee press-ups are an easier version, and “fifty press-ups” means the whole set, not one movement.