Formal:
* Scheduled meeting
* Prearranged meeting
* Appointed meeting
* Planned meeting
* Convened meeting
* Set meeting
Informal:
* Get-together
* Gathering
* Rendezvous
* Hook-up (for casual meetings)
Specific situations:
* Interview (for job interviews)
* Consultation (for professional advice)
* Conference (for larger meetings)
* Summit (for high-level meetings)
The best synonym will depend on the specific context of the meeting.