backflow

1. The flow of water or other liquids, mixtures, or substances into the distributing pipes of a potable supply of water from any other than its intended source. Also see back siphonage
2. Any flow in a direction opposite to the natural or intended direction of flow.

Abnormal backward flow of fluids; regurgitation. See also reflux.

• pyelovenous b. — drainage from the renal pelvis into the venous system occurring under certain conditions of back pressure.

The noun has one meaning:

Meaning #1: a flow that returns toward its source
  Synonym: backflowing