Not a silly question at all. Something that I find quite interesting.
Technically . . . here is a link to the Wikipedia page for "focal length" . . .
Focal length - Wikipedia, the free encyclopedia
From what I understand, if you assume parrallel lines going into the front of a simple lens, the rays of light get bent as they pass between the air to glass and then the glass to air change in medium. The focal length is where these rays of light converge. (As in the 1st diagram on the Wikipedia page at the right.)
I guess with complex lenses, unless you are designing the lens, its hard to know where the "center" of the lens is.
I once read somewhere that the term "focal length" came from Greek? and means "fire length" of a lens in which one of the earlier uses of a lens, before they were used for cameras, was to start fires. And the focal length was how far away you had to hold the lens from the kindling.
The practical definition of focal length in terms of photography are mentioned in the link above as well.
Take care,
Glen