General:
* Meeting
* Gathering
* Appointment
* Assembly
* Confluence
* Tryst (romantic)
Specific:
* For a secret meeting: assignation, clandestine meeting
* For a romantic meeting: date, tryst, assignation
* For a social meeting: gathering, party, get-together
* For a military meeting: assembly, muster, rally
The best synonym depends on the specific situation and the nuance you want to convey.