Formal:
* Assembly: A gathering of people for a specific purpose.
* Convocation: A formal gathering of people, often for a special occasion.
* Conference: A formal meeting for discussion or exchange of information.
* Session: A period of time during which a group meets.
* Summit: A high-level meeting of leaders.
* Congress: A formal assembly of representatives.
* Convention: A large gathering of people with shared interests.
* Symposium: A meeting for the discussion of a particular topic.
Informal:
* Get-together: A casual gathering of friends or family.
* Gathering: A group of people assembled for a common purpose.
* Rendezvous: A planned meeting, often secret or romantic.
* Huddle: A brief meeting for discussion or planning.
* Chat: A casual conversation.
Specific:
* Consultation: A meeting to discuss a problem or issue.
* Interview: A meeting for the purpose of gathering information.
* Negotiation: A meeting to discuss and reach an agreement.
* Trial: A legal meeting to determine guilt or innocence.
The best synonym for "meeting" will depend on the specific context.