Alchemy is a form of magic with almost scientific foundations. Evolving from the archetype of the witch at her cauldron, alchemy is based on the idea that you can mix together various ingredients and dissolve them together to produce potions, elixirs, ointments, unguents and other chemical compounds that will produce magical effects. In the real world, alchemy was a pseudo-science that was legitimately studied, blending foundational chemistry with philosophical beliefs about the spiritual nature of the universe and the ways in which spirit could influence physical matter.
Alchemy is usually more of a background element in fantasy settings; the use of alchemy in the middle of a battlefield is somewhat counter-intuitive to the idea of requiring slow, deliberate blending, stirring, heating and cooling. Instead, alchemy is usually used as a way to explain the existence of constructs and magitek. Still, an alchemist class has sometimes reared its head, most commonly as either a chemist version of an artificer (if not an outright artificer theme), or a kit for the wizard.