It's the opposite of "bilingual" (speaking two languages) or "multilingual" (speaking multiple languages).
For example:
* A person who only speaks English is unilingual.
* A person who speaks both English and Spanish is bilingual.
* A person who speaks English, Spanish, and French is multilingual.
It's important to note that the term "unilingual" is not commonly used. People usually prefer to describe someone's language skills in terms of the number of languages they speak, rather than focusing on what they *don't* speak.