For a romantic relationship, especially a secret one:
* Liaison: This is the most common word for an affair, meaning a romantic relationship outside of a committed relationship.
* Aventure: This word is more general and can refer to any kind of adventure, including a romantic one. It suggests excitement and perhaps a lack of commitment.
For a non-romantic event or situation:
* Affaire: This word is the direct translation of "affair" and can refer to any event or situation.
Other possibilities:
* Relation: This word can refer to a romantic relationship in general, but it doesn't necessarily imply secrecy or lack of commitment.
* Romance: This word is more general and can refer to any romantic relationship, not just an affair.
The best choice for you will depend on the specific context of your situation.