Noun

1. the backwards flow of any fluid
   • 1719- Daniel Defoe, Robinson Crusoe

     ...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...

2. a technique, using a reflux condenser, allowing one to boil the contents of a vessel over an extended period

Verb

1. To boil a liquid in a vessel having a reflux condenser