At Handy-Office, we use the term Leaflet to mean a single piece of paper or card, printed on both sides, and creased or folded.

The size refers to the entire sheet when laid flat. For example, an A4 leaflet is a single sheet of A4, which may be folded into 3 to make a 6-page trifold menu, or folded in half to make a 4-page A5 invitation.