Peristalsis

A wavelike sequence of muscular contraction and relaxation that passes along a tubelike structure, such as a vertebrate intestine, or the body of an earthworm

Peristalsis is a process of contraction and relaxation of the muscle around parts of the alimentary canal.

A simplified image showing peristalsis

The most familiar example of this is in the oesophagus. In the oesophagus, peristalsis pushes the ball of food down the oesophagus. The smooth muscle of the oesophagus contracts and relaxes in sequence, and this pushes the ball of food down the oesophagus.

The muscle is worked by a nervous mechanism called the "migrating motor complex". This works in one direction only, and also works in the lower regions of the digestive system.