The term multilingual can refer to an individual speaker who uses two or more languages, a community of speakers in which two or more languages are used, or speakers of different languages.

Multilingual speakers outnumber monolingual speakers in the world's population.

Multilingual individuals

A multilingual person, in a broad definition, is one who can communicate in more than one language, be it actively (through speaking, writing, and/or signing) or passively (through listening, reading, and/or perceiving). More specifically, the terms bilingual and trilingual are used to describe comparable situations in which two or three languages are involved. A generic term for multilingual persons is polyglot.

Multilingual speakers have acquired and maintained at least one language during childhood, the so-called first language (L1). The first language (sometimes also referred to as the mother tongue) is acquired without formal education, by mechanisms heavily disputed. Children acquiring two languages in this way are called simultaneous bilinguals. Even in the case of simultaneous bilinguals one language usually dominates over the other. This kind of bilingualism is most likely to occur when a child is raised by bilingual parents in a predominantly monolingual environment. It can also occur when the parents are monolingual but have raised their child or children in two different countries.

Definition of multilingualism

One group of academicsFact: date=November 2008 argues for the maximal definition which means speakers are as sufficient in one language as they are in others and have as much knowledge of and control over one language as they have of the others. Another group of academics argues for the minimal definition, based on use. Tourists, who successfully communicate phrases and ideas while not fluent in a language, may be seen as bilingual according to this group.

However, problems may arise with these definitions as they do not specify how much knowledge of a language is required to be classified as bilingual. As a result, since most speakers do not achieve the maximal ideal, language learners may come to be seen as deficient and by extension, language teaching may come to be seen as a failure. One does not expect children to "speak chemistry" or to have become a professional athlete by the time they have left school, yet for graduating school children anything less than fluency in a second language could be seen as inadequateFact: date=November 2008.

Since 1992, Cook has argued that most multilingual speakers fall somewhere between minimal and maximal definitions. Cook calls these people multi-competent.

Learning language

A broadly held, yet nearly as broadly criticisedFact: date=November 2008, view is that of the United States linguist Noam Chomsky in what he calls the human 'language acquisition device '— a mechanism which enables an individual to recreate correctly the rules (grammar) and certain other characteristics of language used by speakers around the learner.Santrock, John W. (2008). Bilingualism and Second-Language Learning. A Topical Approach to Life-Span Development (4Th ed.) (pp. 330-335). New York, NY: McGraw-Hill Companies, Inc. This device, according to Chomsky, wears out over time, and is not normally available by puberty, which he uses to explain the poor results some adolescents and adults have when learning aspects of a second language (L2).