An Alchemist is someone who practices Alchemy.
Alchemy was a dead end area of work. It included trying to create the Elixir of Life, and making precious metals from more common ones. The Elixir of Life granted health and immortality to anyone who drinks it, however it was never created. Alchemists in China did accidentally create gunpowder, a very useful substance for many centuries. They also looked for the Philosophers Stone, as stated above, which turns regular metals into gold. Alchemy did pave the way towards chemistry, and discovered many of the basic chemistry principles.