General:
* Covert: This is a common word for something done secretly, often with the implication of a hidden agenda or illegal activity.
* Secret: This is the most direct word for something kept hidden.
* Stealthy: This implies moving or acting secretly and quietly.
* Surreptitious: This implies doing something secretly and cautiously, often to avoid detection.
* Underhanded: This implies doing something secretly and dishonestly.
More specific:
* Clandestine: This implies a meeting or operation that is secret and illegal.
* Furtive: This suggests a secretive and suspicious manner.
* Subversive: This implies secret activity intended to undermine or overthrow authority.
Figurative:
* Behind closed doors: This refers to a meeting or event that is kept secret.
* On the sly: This implies doing something secretly and dishonestly.
The best word for your specific situation will depend on the context and the meaning you want to convey.