Wikipedia
Pars pro toto, Latin for "a part (taken) for the whole", is a figure of speech where the name of a portion of an object, place, or concept represents its entirety. It is distinct from a merism, which is a reference to a whole by an enumeration of parts; metonymy, where an object, place, or concept is called by something or some place associated with the object, place, or concept; or synecdoche, which can refer both to this and its inverse of the whole representing a part.
In the context of language, pars pro toto means that something is named after a part of it, or after a limited characteristic, in itself not necessarily representative for the whole. For example, "glasses" is a pars pro toto name for something that consists of more than just two pieces of glass. Pars pro toto is a common device in iconography, where a particular icon can stand for a complete set of characteristics. James George Frazer used "pars pro toto" to explain his concept of contagious magic. Examples of common pars pro toto usage in political geography include " Russia" or " Russians", for the entire former Russian Empire or former Soviet Union or its people, Taiwan or Taipei ("Chinese Taipei") for Republic of China, Holland for the Netherlands, and, particularly in languages other than English, using the translation of " England" in that language for " United Kingdom of Great Britain and Northern Ireland". Among English-speakers " Great Britain" is a common pars pro toto shorthand for the entire United Kingdom. Switzerland's name (in German Schweiz) comes from its central Canton of Schwyz.
The inverse of a pars pro toto is a totum pro parte, in which the whole is used to describe a part, such as widespread use of " America" (which originally named the entire western hemisphere to place it geographically, with alliteration, alongside Asia, Africa, "Europa," and ultimately Arctica, Antarctica and Australia) in place of " United States of America", "United States" or "USA". The term synecdoche is used for both, as well as similar metaphors, though in Greek it literally means "simultaneous understanding".
Pars pro toto (and totum pro parte) can be imprecise, controversial or even offensive. In the UK, for instance, substituting "Great Britain" or using the terms "British" or "English" for countries or provinces (that choice of terms is a point of contention all its own) which are neither British nor English can be a hot button issue and divide along sectarian lines, from a grand political perspective down to an individual level of personal preference of how one wishes to self-identify. In Northern Ireland especially, many Unionists may wish to be referred to as British or even English and may prefer to call the region Ulster, whereas Irish Republicans are not offended by the "Irish" identifier but may well take umbrage to being called English or referring to an incomplete Ulster or declare it part of Britain when referring to the province to emphasize its divided nature.