It's a type of science built on the concept of equivalent exchange. Sort of a magic in its own ways. Through the use of a transmutation circle, an alchemist can rapidly change raw materials into another shape. This can be for simple things like, say, turning a bunch of boards and nails into a house. Or if you're really good, you can use it to rapidly change the environment around you to attack. Almost like bending, kind of. Only it's all based on science.