Common body weight exercise for the abdominal muscles.
The basic crunch is done lying on the back bending the legs, arms sideways with the hands supporting the head. By contracting the abdominal muscles the upper body is moved up until the shoulder blades no longer touch the ground.
There are several variations to the basic crunch focusing on different muscles.
Lifting the complete upper body up from the ground is a sit up. Sit ups and crunches are however often confused or treated synonymous.