sit up

Short definition from Wiktionary

Verb

1. 1
   To assume a sitting position from a position lying down.
2. 2
   To sit erect.
3. 3
   To show sudden interest or surprise.
4. 4
   To bounce, especially to a comfortable height.
5. 5
   To not go to bed (notionally remaining in a sitting position).
6. 6
   For a young lady to open her house for several successive nights for any visitors who wish to pay their respects.

Noun

1. 1
   Alternative spelling of sit-up.
2. 2
   An exercise in which the abdominal muscles are contracted, bringing the torso of a person lying down to a more upright position. The degree of difficulty can be controlled by choosing from partial-range or full-range versions.