What does "the alchemist" mean in Coelho's The Alchemist?
An alchemist was a person who sought, among other things, to turn base metal into gold. Santiago meets an alchemist, a veiled stranger on a white horse, at the oasis in the desert. Santiago travels with the alchemist for a time and witnesses him actually turning lead into gold. Santiago understands that you cannot become a successful alchemist without first purifying your soul.
Santiago learns to trust the alchemist, but the alchemist tells him he can't tell him the secret of turning metal into gold, because it "was my Personal Legend, not yours." He tells Santiago he should go back to the oasis, and Santiago does. He has gleaned from the alchemist that he has to faithfully pursue his own Personal Legend with a pure heart. In this sense, Santiago also becomes a metaphorical alchemist. He takes all he has learned on his journeys and is able to turn it into "gold" by returning home again to find his treasure.
An alchemist is a person who studies alchemy. Alchemy is a combination chemistry science and a philosophy both. The study seeks to change base metals into gold as well as find a cure for all diseases. In addition, it seeks to extend the life span of humans. In a way, it is an ideal, a goal toward which many strive, but no one achieves. The goal itself is unattainable because one would have to be the Creator to reach these goals. In literature, it can refer to anyone who seeks to attain the unobtainable much like Don Quixote who dreamed the impossible dream.