Real Dictionary

sit up
phrasal verb: If you sit someone up, you move them into a sitting position when they have been leaning back or lying down.
phrasal verb: If you sit up, you move into a sitting position when you have been leaning back or lying down.

sit-up
Sit-ups are exercises that you do to strengthen your stomach muscles. They involve sitting up from a lying position while keeping your legs straight on the floor.
to raise (oneself or another) from a recumbent to an upright or alert sitting posture
noun: a physical exercise in which the body is brought into a sitting position from one lying on the back

sit-up in British English
an exercise in which a person lying flat on the back rises to a sitting position without using the hands: sit-ups are usually done in series
to rise from a supine to a sitting position
to rise to a sitting position
noun
noun: an exercise in which a person lies flat on the back, lifts the torso to a sitting position, and then lies flat again without changing the position of the legs: formerly done with the legs straight but now usually done with the knees bent