Denotation, as defined, is a kind of reference. Thus, the term "mermaid" refers but does not denote, since there are no mermaids, while the term "king" denotes since there are kings. Note though that terms like "mermaid" are often taken to represent the empty set.

Also, there is no uniformity about usage of the terms "reference" and "denotation": Some confuse them; some use them differently. As I use the term, the term "mermaids" refers to mermaids, but does not denote, if we are speaking about ordinary physical reality, since there are no mermaids.