General meaning:
* Gathering
* Assembly
* Meeting
* Conference
* Summit
* Forum
* Colloquium
* Symposium
Meaning: established practice or custom:
* Tradition
* Custom
* Norm
* Standard
* Rule
* Protocol
* Etiquette
Meaning: agreement or contract:
* Treaty
* Pact
* Accord
* Agreement
* Compact
Meaning: a group of people sharing a common interest:
* Society
* Association
* Guild
* Fraternity
* Sorority
The best synonym will depend on the specific context of your sentence.