Ardent spirits (ethyl alcohol), in alchemy, are those liquors obtained after repeated distillations from fermented vegetables. They are thus called because they will take fire and burn. Examples include brandy, spirits of wine, etc.
References
This article incorporates text from a publication now in the public domain: Chambers, Ephraim, ed. (1728). "Ardent spirit". Cyclopædia, or an Universal Dictionary of Arts and Sciences (1st ed.). James and John Knapton, et al. - "Ardent spirit". The Incompleat Chymist. Archived from the original on 2005-12-22. Retrieved 2006-01-22.