It depends on the specific meaning:
* If "bilingual" refers to speaking two languages:
* Monolingual: This is the most common and direct antonym, meaning speaking only one language.
* Unilingual: Less common, but also means speaking only one language.
* If "bilingual" refers to being fluent in two languages:
* Unfluent: This emphasizes the lack of fluency, but doesn't necessarily mean they only speak one language.
* Limited: This suggests a lack of proficiency, not necessarily limited to one language.
* If "bilingual" refers to a text or resource being in two languages:
* Monolingual: This describes a text or resource only in one language.
* Single-language: This is a more specific alternative to "monolingual."
Beyond direct antonyms:
* Unilingual: This can also be used to describe someone who only speaks one language, even if they understand another language.
* Multilingual: Technically not an antonym, but it contrasts with "bilingual" by implying fluency in multiple languages, more than two.
The best choice depends on the specific context and the meaning you want to convey.