I’ll take this definition suggested by William C. Knott, author of The Craft of Fiction (which I think we studied in my university senior seminar, though I don’t recall my or the professor’s final verdict on it):
Almost anyone can write; only writers know how to rewrite. It is this ability alone that turns the amateur into the professional.
(Interesting conundrum: when I searched for the book title, its author appeared as Percy Lubbock in the majority of search results. This refers to the book published in 1921. William Knott published The Craft of Fiction: Revised Edition in 1973, but I don’t know how much the two books are related — or even if the attribution to William Knott is one of those widely dispersed internet myths. Then again, the Google Books copy of Lubbock’s book doesn’t contain the word “amateur.”)