Pars pro toto is Latin for synecdoche, or "(taking) a part for the whole". When used in a context of language it means that something is named after a part of it (or after a limited characteristic, in itself not necessarily representative for the whole). E.g. "glasses" is a "pars pro toto" name for a complicated structure to put on a human's nose, and implying also other materials than simply two pieces of glass.