Wikipedia: "A
pancake lens is colloquial term for a flat, thin lens (short barrel), generally a normal or slightly wide prime lens for a camera."
Here we are offered two definitions:
terminology - What is a pancake lens? - Photography - Stack Exchange:
"Pancake lens is a lens which is designed to be
physically very thin and compact, relatively pocketable when combined with a slim body."
"Pancake lens" is a purely physical description. If the lens is significantly wider than it is long, it's a pancake lens. Actual overall size has nothing to do with it, and it has no implications about image performance, aside from the fact that it's generally very hard to pack a telephoto or fast prime into the pancake form-factor. It's worth noting that a "Pancake lens" does
not have to be small or compact, and a small lens is not automatically a pancake lens. It's only a description of the lens form-factor."
You can guess that the scientist in me prefer the second of them since it is more exact and therefore more useful.
Strange enough, I cannot find a definition at camerapedia.
This blog, while not offering a definition, suggests that they should instead have been called
"hockey puck" lenses. He has got a point there.