"An index whose columns are used in the WHERE clause and also are the only columns returned in the SELECT clause."

Not quite. A better answer, IMO, would be: "A non-clustered index that contains every column from the indexed table that is used by the query."

The answer given makes it sound as if no other tables can be joined and still take advantage of a covering index. That is untrue. Also, covering indexes are always non-clustered.