What is a single lense?
A single lense is just one piece of glass (or other material), bounded by at least one curved surface.
Most photographic "lenses", or "lenses" in other optical devices, are made of multiple pieces of glass. Actually they should be called objectives (or oculars if at the eye-side of e.g. a telescope).
A single lens has all kinds of abberations, so it will not form a perfect image. That's why they are often combined.