If you ask 5 people what Alchemy is they will give you 10 different meanings.  Here we are using Alchemy to mean a mixture that is magically greater than the sum of its parts.

Or... for the nerdy: Any creation that due to mixture or magick has different properties than the ones expressed by the original components.

A great example is an fluid condenser.  By themselves each herb would simply provide its essence or purpose, but when ritually put together in the structure of the spell, the condenser expresses the essence of an entire element.