Monoglottism (Greek μόνος monos, "alone, solitary", + γλῶττα glotta, "tongue, language") or, more commonly, monolingualism or unilingualism, is the condition of being able to speak only a single language, as opposed to multilingualism. In a different context, "unilingualism" may refer to a language policy which enforces an official or national language over others.

Being monolingual or unilingual is also said of a text, dictionary, or conversation written or conducted in only one language, and of an entity in which a single language is either used or officially recognized (in particular when being compared with bilingual or multilingual entities or in the presence of individuals speaking different languages). Note that monoglottism can only refer to lacking the ability to speak several languages. Multilingual speakers outnumber monolingual speakers in the world's population.

Suzzane Romaine pointed out, in her 1995 book Bilingualism, that it would be weird to find a book titled Monolingualism. This statement reflects the traditional assumption that linguistic theories often take on: that monolingualism is the norm. Monolingualism is thus rarely the subject of scholarly publications, as it is viewed to be an unmarked or prototypical concept where it has the sense of being normal and multilingualism is the exception.

The assumption of normative monolingualism is also often the view of monolinguals who speak a global language, like the English language. Crystal (1987) said that this assumption is adopted by many in Western society. One explanation is provided by Edwards, who in 2004 claimed that evidence of the "monolingual mindset" can be traced back to 19th century Europe, when the nation was rising and a dominant group had control, and European mindsets on language were carried forth to its colonies, further perpetuating the monolingual mindset.

Another explanation is that the nations who speak the English language are both “the producers and beneficiaries of English as a global language” and the populations within these countries tend to be monolingual.