I've noticed that when I google "classic guitar" or "classical guitar," in addition to what I'm looking for, I get a number of hits having to do with such things as "the 'classic' Fender Stratocaster" and such. This is a very broad usage of the word "classic," and it seems that a few people, including "GUEST" just above, have made this assumption about what constitutes a "classic" guitar.

A classic guitar, in the way the word "classic" is usually meant in this context, is an acoustic flat-top guitar with a flat fingerboard 2 inches wide at the nut, a slotted headstock, and which is strung with nylon strings. Like almost any musical instrument, one can play all kinds of music on a classic guitar, but its primary usage is for playing classical music (works of such composers as Bach, Tarrega, Sor, Carulli, lute transcriptions, etc.). It is an excellent instrument for song accompaniment, and as indicated in this thread, many singers of folk songs prefer to use a classic guitar.

A typical classic guitar.

I realize, of course, that I am fighting a losing battle against the tendency to fuzz out the commonly agreed upon (dictionary) definitions of various words to make them mean anything one wants them to mean.

Don Firth