Formal:
* Congress: A formal meeting of representatives from different countries or organizations.
* Summit: A high-level meeting of leaders or experts.
* Convention: A large gathering of people for a specific purpose, often with presentations, workshops, and networking.
* Symposium: A formal meeting where experts discuss a particular topic.
* Forum: A public meeting or discussion, often on a specific issue.
Informal:
* Meeting: A gathering of people for a specific purpose, usually less formal than a conference.
* Gathering: A general term for a group of people coming together.
* Assembly: A group of people gathered together for a specific purpose, often with a formal agenda.
* Session: A period of time devoted to a specific activity or discussion.
* Conclave: A secret or private meeting.
Other:
* Parley: A discussion or negotiation, especially between enemies.
* Council: A group of people assembled to advise or govern.
* Consultation: A meeting for advice or discussion.
The best word to use will depend on the specific context. For example, if you are talking about a meeting of scientists to discuss research findings, "symposium" would be a good choice. If you are talking about a meeting of business leaders to discuss industry trends, "conference" would be a more general term.