To assume a sitting position from a position lying down.

“Despite being sick, Lorin sat up to greet the visitors.”

To sit erect.

“Sit up straight, mister!”

To show sudden interest or surprise.

“He sat up when we mentioned the increased pay package.”

An exercise in which the abdominal muscles are contracted, bringing the torso of a person lying down to a more upright position.

“Susan's doing sit-ups to strengthen her muscles.”