• Bilingualism
• Code Switching
• Contact Linguistics
• Diglossia
• English-Only Movement
• Home Language
• Linguicism
• Native Language (L1) and Second Language (L2)

• Sociolinguistics
• Speech Community

Examples and Observations:

• "We estimate that most of the human language users in the world speak more than one language, i.e. they are at least bilingual. In quantitative terms, then, monolingualism may be the exception and multilingualism the norm. . . .
  
  "[I]t is a reasonable assumption that the marginal role research on multingualism has played within linguistics until some decades ago is a result of the monolingual bias of (particularly) European thinking about language which came into being during a phase of European history in which the nation states defined themselves not in the least by the one (standard) language which was chosen to be the symbolic expression of their unity. . . .
  
  "[I]t can be argued that what we perceive as the problems surrounding multilingualism today are to a large degree a consequence of the monolingualism demanded, fostered and cherished by the nation states in Europe and their knock-offs around the world."
  (Peter Auer and Li Wei, "Introduction: Multilingualism As a Problem? Monolingualism As a Problem?" Handbook of Multilingualism and Multilingual Communication. Mouton de Gruyter, 2007)

• Bilingualism and Multilingualism
  "Current research . . . begins by emphasizing the quantitative distinction between multilingualism and bilingualism and the greater complexity and diversity of the factors involved in acquisition and use where more than two languages are involved (Cenoz 2000; Hoffmann 2001a; Herdina and Jessner 2002). Thus, it is pointed out that not only do multilinguals have larger overall linguistic repertoires, but the range of the language situations in which multilinguals can participate, making appropriate language choices, is more extensive. Herdina & Jessner (2000b:93) refer to this capacity as 'the multilingual art of balancing communicative requirements with language resources.' This wider ability associated with the acquisition of more than two languages has also been argued to distinguish multilinguals in qualitative terms. One . . . qualitative distinction seems to lie in the area of strategies. Kemp (2007), for example, reports that multilingual learners' learning strategies differ from those of monolingual students learning their first foreign language."
  (Larissa Aronin and David Singleton, Multilingualism. John Benjamins, 2012)
• Are Americans Lazily Monolingual?
  "The celebrated multilingualism of not just Europe but also the rest of the world may be exaggerated. The hand-wringing about America’s supposed linguistic weakness is often accompanied by the claim that monolinguals make up a small worldwide minority. The Oxford linguist Suzanne Romaine has claimed that bilingualism and multilingualism 'are a normal and unremarkable necessity of everyday life for the majority of the world’s population.'
  
  "But the statistics tell a murkier story. Recently, the Stockholm University linguist Mikael Parkvall sought out data on global bilingualism and ran into problems. The reliable numbers that do exist cover only 15 percent of the world’s 190-odd countries, and less than one-third of the world’s population. In those countries, Mr. Parkvall calculated (in a study not yet published), the average number of languages spoken either natively or non-natively per person is 1.58. Piecing together the available data for the rest of the world as best he could, he estimated that 80 percent of people on the planet speak 1.69 languages--not high enough to conclude that the average person is bilingual.
  
  "Multilinguals may outnumber monolinguals, but it’s not clear by how much. The average American may be no more monolingual or less multilingual than any other average person elsewhere on the planet. At the very least, we can’t say for sure--not in any language."
  (Michael Erard, "Are We Really Monolingual?" The New York Times Sunday Review, January 14, 2012)
• New Multilingualisms
  "[I]n paying attention to the language practices of young people in urban settings, we see new multilingualisms emerging, as the young people create meanings with their diverse linguistic repertoires. We see the young people (and their parents and teachers) using their eclectic array of linguistic resources to create, parody, play, contest, endorse, evaluate, challenge, tease, disrupt, bargain and otherwise negotiate their social worlds."
  (Adrian Blackledge and Angela Creese, Multilingualism: A Critical Perspective. Continuum, 2010)