Now, Merriam-Webster says it’s two words, copy editor, but The Chicago Manual of Style says it’s one, and they are both considered authoritative in my line of work, so really you get to choose. I have decided to go with one because that’s how such compounds generally trend: they start out as two words (base ball) and eventually become one, sometimes with an awkward hyphenated period in between (for a while it was base-ball). Also, even Merriam-Webster acknowledges that the verb is to copyedit.