This document defines and discusses the concepts of multilingualism, bilingualism, and diglossia. It states that multilingualism is the ability to communicate effectively in three or more languages. Bilingualism is defined as the use of two languages or language codes, where speakers must master both their native language and a foreign language. Diglossia refers to a situation where two languages or language varieties are used in different contexts by the same speakers, such as a high (H) variety used in formal situations and a low (L) variety used in informal situations.