Alchemy in games is patterned after the real-life proto-science of the same name. Alchemy concerned itself with many of the same issues as the modern science of chemistry, but also incorporated significant philosophical and mystical components, and rather than being widely shared and reviewed, alchemical discoveries were often made in secret and recorded using some of the earliest forms of cryptography.
In games that have actual, functioning magic, alchemical items are often not-quite magical, being a mixture of herbalism, chemistry, and yes, some magic. Alchemical items often include potions and oils, or unstable concoctions that can be thrown at targets.
This tag should be applied to questions about alchemy itself, the production or use of alchemical items. It should not be used for questions that are simply about a class called alchemist: for such questions the alchemist tag should be used.