Let's review

Only employees of the Wikimedia Foundation
INCORRECT! Wikimedia Foundation employees can be volunteer editors, but only if they want to. They also have to declare their association with the Wikimedia Foundation to avoid any conflict of interest.
A trained expert
INCORRECT! A trained expert can be an editor, but volunteers who are not experts are editors as well.
Anyone who decides to edit a Wikimedia project
CORRECT! Wikimedia projects strive to be open and inclusive. Anyone who edits a Wikimedia project, like Wikidata, is considered an editor.
Only someone who has taken a course in editing a Wikimedia project
INCORRECT! You do not have to take a course in order to be considered an editor. (Although knowing how edit correctly is always a good idea!)

Table of Contents