Societal bilingualism (or multilingualism) denotes the characteristic linguistic situation in a particular speech community, i.e. in a particular society or nation in which more than one language is used. In this connection we can further distinguish between official multilingualism and de facto multilingualism. The former indicates that the use of more than one language in a speech community is officially recognized. The respective languages are acknowledged by the nation's constitution and therefore have the status of an official language. The latter refers to the difference between what is officially stated and what is the actual linguistic situation in a nation. For example, in (officially) monolingual or bilingual speech communities other languages without official status are also used. This makes these communities in fact multilingual in nature. Canada is bilingual with the official languages English and French. However, there are also other languages used in this nation which are not officially recognized (e.g. the indigenous languages of Inuit and Indian citizens). Therefore Canada is officially a bilingual nation while it is de facto a multilingual country. Likewise, Switzerland is officially a multilingual nation and official documents for the entire country are written in French, German, and Italian. Each canton, however, has its own official language and is a rather monolingual environment to grow up in.
(Information adapted from Clyne, 1997)
The terms bilingualism and multilingualism used in connection with a linguistic characterization of a particular speech community are neutral terms in that they do not imply a hierarchy, i.e. a difference in social status or between the languages used in this community. This is different when diglossia is considered. In diglossic communities the language varieties typically differ in prestige with one variety being the high variety (H-variety) and another variety being the low variety(L-variety).
Read more: e.g. Clyne, M. (1997). Multilingualism. In: Coulmas, F. (ed.). The Handbook of Sociolinguistics (Oxford/UK and Malden/USA: Blackwell), 301- 314.