[pkd]

Eh, no? It's based on the field of view, not on the physical length of the lens.

Fuji's 27mm pancake lens is only 23mm tall. A telephoto lens it is not.

[roelschroeven]

It's the original/technical definition. In common parlance telephoto means small field of view.

The commonly used meaning has drifted from the original/technical definition. The same has happened with other things. 'Lens' for example, is technically one single element; the whole assembly is technically an 'objective'. But in photography the latter is what is commonly referred to as 'lens'.

Language isn't always precise and often context dependent (I don't always like that but there's not a lot I can do).

[sprobertson]

GP was more correct in that it's the construction (using a telephoto group) that makes it shorter. Being shorter does not in itself make it telephoto.