in the context of
photography, any
film format whose width is smaller or equal to
135, usually excluding
miniature formats like the
Minox 8mm (which are not very common).
Currently, the major sellers in small format (after 135, of course) are the APS (Advanced Photo System) and the 110 cartridge film.
Among the historically relevant, we find the 126 (again, a cartridge) and the Kodak disc (not a huge sales success).
The main problem with small format film (more so with films smaller than 135) is that grain limits enlargeability.
So, the picture that looks lovely as 4"x5" print may reduce itself to a mosaic of grain when blown up to poster size. Also, any small scratch runs a good chance of destroying the picture - and film, being basically gelatine, scratches easily, especially when wet (that's to say, during processing).
Enlargeability and image quality not being such a major concern to the average consumer level photographer (he of the 5 rolls/year tops), small format negative film makes up for about 90% of the world film market. Otherwise we would all be shooting 120.