Contrary to what is often believed, most of the world's population is bilingual or multilingual. Monolingualism is characteristic only of a minority of the world's peoples. According to figures cited in Stavenhagen (1990) for example, five to eight thousand different ethnic groups reside in approximately 160 nation states. Moreover, scholars estimate that there are over 5000 distinct languages spoken in that same small number of nation states. What is evident from these figures is that few nations are either monolingual or mono-ethnic. Each of the world's nations has groups of individuals living within its borders who use other languages in addition to the national language to function in their everyday lives.

Definitions of Bilingualism and Multilingualism

When people hear the term bilingual many imagine an individual who speaks two languages perfectly. For them someone who is 'truly' bilingual is two native speakers in one. They imagine that such a person can speak, understand, read, and write in two languages at the highest levels. For others, the term bilingual means something quite different. When newly arrived immigrant children entering U.S. schools, for example, are described as 'bilingual children,' the term is often used as a euphemism for 'poor' and 'uneducated'. In this case, newly arrived immigrant children do not yet function in two languages. They are monolingual speakers of their first language and not bilingual at all. The term bilingual here is used to convey a very different set of meanings from what linguists intend.

Defining Multilingualism

The question of how to define bilingualism or multilingualism has engaged researchers for a very long time. Some researchers have favored a narrow definition of bilingualism and argued that only those individuals who are very close to two monolinguals in one should be considered bilingual.
More recently, however, researchers who study bilingual and multilingual communities around the world have argued for a broad definition that views bilingualism as a common human condition that makes it possible for an individual to function, at some level, in more than one language. The key to this very broad and inclusive definition of bilingualism is 'more than one'.
From the perspective of this framework, a bilingual individual is not necessarily an ambilingual (an individual with native competency in two languages) but a bilingual of a specific type who, along with other bilinguals of many different types, can be classified along a continuum. Some bilinguals possess very high levels of proficiency in both languages in the written and the oral modes. Others display varying proficiencies in comprehension and/or speaking skills depending on the immediate area of experience in which they are called upon to use their two languages.
According to this perspective, one admits into the company of bilinguals individuals who can, to whatever degree, comprehend or produce written or spoken utterances in more than one language. Thus, persons able to read in a second language (e.g. French) but unable to function in the spoken language are considered to be bilinguals of a certain type and placed at one end of the continuum. Such persons are said to have receptive competence in a second language and to be 'more bilingual' than monolinguals who have neither receptive nor productive abilities in a language other than their first. The judgment here is comparative: total monolingualism versus a minor degree of ability to comprehend a second language.

Types of Bi- and Multilinguals

Because there are very different kinds of bilinguals and multilinguals, much effort in the study of bilingualism has gone into developing categories which might make the measurement and description of these differences possible. The categories used to describe different types of bilinguals reflect different researchers' interests in focusing on specific aspects of bilingual ability or experience. Researchers concerned about the age of acquisition of bilingualism, for example, classify bilingual individuals as either early or late bilinguals and further subdivide early bilinguals into simultaneous bilinguals (those who acquired two languages simultaneously as a first language) or sequential bilinguals (those who acquired the second language (L2) after the first language (L1) was acquired). Researchers, on the other hand, concerned about the differences between persons who choose to study a second language and those who grow up in communities where several languages are spoken have used the terms elite, academic, and elective bilinguals for the former and natural, folk, and circumstantial bilinguals for the latter.
The usefulness of these labels and categories clearly depends on the specific interest a researcher has in bilingualism. Meaningful comparisons of bilingual persons cannot generally be made unless attention is given to the differences and similarities between these individuals in terms of a number of key dimensions such as age of acquisition of the second language, circumstances in which the two languages are used, patterns of use of the two languages in the surrounding community, level of formal education received in each language, and degrees of proficiency.
by Guadalupe Valdés

Suggested Readings

Appel, R, and P. Muysken. 1987. Language contact and bilingualism. London: Edward Arnold.
Grosjean, F. 1982. Life with two languages. Cambridge, MA: Harvard University Press.
Hamers, J. F., and M. H. A. Blanc. 1989. Bilinguality and bilingualism. Cambridge, UK: Cambridge University Press.
Weinreich, U. 1974. Languages in contact. The Hague: Mouton.
Romaine, S. 1989. Bilingualism. Oxford: Basil Blackwell.

http://www.linguisticsociety.org/resource/multilingualism