Multilingualism is a situation in which a person speaks two or more languages. The case of using two languages is considered separately, it is called bilingualism and has been of great concern to linguistics and education. However, now even bilingualism is considered a special case of multilingualism. Both are contrasted with monolingualism and considered side by side.
The Language contact, which presupposes contact between different groups, is at the basis of bilingualism and multilingualism.
Multilingualism arises in situations where different groups choose (or are forced) to coexist. In fact, historically, there have been various powerful languages that functioned at times as trade languages or lingua franca, such as today’s international languages. In some cases, mixed languages or pidgins were created, which began as attempts at mutual understanding between people who had no common language. This phenomenon flourished mainly during the colonial period and complex languages mainly emerged from a strong European and an indigenous language. Then, when these languages acquire native speakers, they are called creoles. They expand, developing more complex grammar and a wider vocabulary to meet needs that are no longer limited to transactions. The current Haitian Creole language, for example, is native to modern Haitians but is historically based on a mixed language (based on French and the local language).
Source: Dictionary of Common Modern Greek