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.