Multilingualism
Multilingualism refers to the state of knowing two or more languages, as opposed to one (monolingualism). People or speech communities - groups of people sharing a way of speaking - can be multilingual (societal multilingualism). Where two languages are used, the term bilingualism is often preferred, and occasionally trilingualism is also used, for three languages.
A multilingual individual need not be completely fluent or a native speaker of either language, nor do they need to regularly use both languages. Multilingualism is the norm in most societies, and frequently speakers may command upwards of three or four different languages. In some cases, the languages may be highly similar, such that speakers of one can readily understand the other, but if culturally they are considered separate tongues, then speakers may still be regarded as multilingual.