While fluency implies a high level of proficiency in a language, bilingual simply means being able to communicate effectively in two languages.
Other related terms include:
* Multilingual: Able to speak more than two languages.
* Polyglot: Someone who speaks many languages.
* Semi-lingual: Someone who has a basic level of proficiency in a second language.
* Proficient: Having a high level of skill in a language.
The best term to use depends on the specific level of proficiency and the number of languages being discussed.