When discussing bilingualism, terms must be used carefully. Everyday ideas of what counts as “bilingual” often differ substantially from linguistic usage.
Monolingualism
In the narrowest sense, a person who commands only one language is monolingual. The difficulty begins as soon as someone has partial knowledge of another language.
Bilingualism
A very strict definition reserves the term for people with native-like command of two languages. Broader definitions focus on regular use, functional competence or the ability to communicate in two languages.
Competence may differ by domain. Someone can discuss family matters fluently in one language but professional topics more easily in another. Reading, writing, speaking and comprehension may also be distributed unevenly.
Multilingualism
Real multilingual repertoires are usually asymmetrical and shaped by biography, context and need. They should not be imagined as several identical monolingual competences inside one person.
First language, mother tongue and dominant language
These expressions are not interchangeable. A first-acquired language may later cease to be dominant. “Mother tongue” can refer to family origin, emotional affiliation or early acquisition and is therefore ambiguous.