Monolingualism is knowing and using just one language. This language will be the speaker’s mother tongue.
Contrast this with bilingualism. where someone knows and uses 2 languages as mother tongues and also multilingualism where someone has more than 2 languages as their mother tongues.
A person who speaks just one language is a monoglot. Although no reliable statistics are available, anecdotal evidence suggests that native born residents of most English speaking countries tend to be more monolingual than people born and raised in non-English speaking countries. (This report suggests 73% of Americans are monolingual.)
Dictionaries can also be defined as monolingual when they are just for one language; again this can be compared to bilingual dictionaries which typically have a word in one language and the associated explanation in another language.
Useful Links
Monolingual Classes & TEFL– all about teaching a monolingual class
Bilingualism – a look at what it entails, including how to become and encourage people to be bilingual
Multilingualism – all about the ability to speak more than one language