An exercise for the stomach muscles. You lie on your back with your knees bent and your feet flat on the floor, then curl your body up until you are sitting, then lie back down. That is one sit-up. People almost always count them, so the word is almost always plural: fifty sit-ups, a hundred sit-ups. The verb you use with it is do — do twenty sit-ups. Not make, not play. Note the hyphen: the noun is sit-up, while the plain verb sit up means to move from lying to sitting ("He sat up in bed"), or to stay awake late ("I sat up until two waiting for her"), or to suddenly pay attention ("That news made everyone sit up"). A close relative is the crunch, a smaller version where only your shoulders come off the floor; gyms and PE teachers now often prefer crunches, because full sit-ups can be hard on the lower back. In daily talk, though, most people still say sit-ups for both.