Formal:
* Debate: A formal discussion with opposing viewpoints.
* Conference: A formal meeting for discussion.
* Symposium: A formal meeting with presentations and discussion.
* Dialogue: A formal conversation between two or more people.
* Discourse: A serious and formal discussion of a subject.
Informal:
* Chat: A casual conversation.
* Talk: A general term for any conversation.
* Conversation: A friendly and informal discussion.
* Exchange: A back-and-forth conversation.
* Confer: To talk about something together.
Other:
* Disputation: A formal argument or debate.
* Parley: A discussion between enemies.
* Consultation: A discussion to seek advice or information.
* Conferment: A formal discussion leading to a decision.
The best word to use will depend on the specific context and the tone you want to convey.