Alchemy has meant multiple things at different times. The alchemies of old dealt with the elixir of life, turning metals into gold, but these are archaic at best.
In our world and age, Alchemy means defining and manipulating the elemental properties of objects.
There are several elements, but the core four are earth, water, air, and fire. These elements correspond with the four known states of matter.
Earth represents solid objects. Warter represents liquid objects. Air represents gaseous objects. Fire represents plasmatic objects.
To change the elemental properties of an object, the amount of mana within it must be changed, for it is through mana that matter exists.