1. The removal of liquids or gas by means of a siphon utilizing gravitational pressure, particularly as a means of emptying a body cavity.
2. In medicine, the use of a siphon, as in gastric lavage or in draining the bladder.

A "gastric lavage" refers to the process of washing out a hollow organ; especially the stomach, by flushing with water.

1. The flowing of used, contaminated, or polluted water back into the potable water system, caused by reduced pressure in a pipe or vessel.
2. The flowing back of used, contaminated, or polluted water from a plumbing fixture or vessel into the pipe which feeds it; caused by reduced pressure in the pipe.

Back siphonage is a result of liquids at a lower level drawing water from a higher level.

Back siphonage should not to be confused with backflow because back siphonage is a result of liquids at a lower level drawing water from a higher level; while backflow is pormoted entirely by pressure in the reservoir itself.

Backflow can't occur through an intermediate high-point while back siphonage flows through an intermediate high-point and so it is much more difficult to guard against.