In the context of alchemy, "essence"- also known as "spiritus" (i.e. the spirit), the "finest" and the "most precious"- could mean a number of different things: it could mean good smelling oils which were distilled from certain plants, or the best and most effective or potent part of a substance. In the case of wine, for example, this "potent" part would be alcohol.

For a comparable symbol, see 'Spiritus (alchemy)'.