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.