I'd think that the distillate what was left after you separated the water out
Scientific jargon making its way back into everyday language, again. Much havoc is created that way.
The latin root of distil(l) actually stems from the drops of condensed vapor which fall into the receiving vessel. So distillate always refers to the component which is separated due to its (higher) volatility. The question of wether it was the useful component is secondary.