reflux

noun

technique involving the condensation of vapors and the return of this condensate to the system from which it originated

Pronunciation: /ˈɹiː.flʌks/

The backwards flow of any fluid.
“[…] after a little way out to sea, there was a current and wind, always one way in the morning, the other in the afternoon. This I understood to be no more than the sets of the tide, as going out or coming in; but I afterwards understood it was occasioned by the great draft and reflux of the mighty river Orinoco […]”
A technique, using a reflux condenser, allowing one to boil the contents of a vessel over an extended period.
The leaking of stomach acid up into the oesophagus.