Formal:
* Gathering: This is a general term for a group of people coming together.
* Assembly: This suggests a formal or organized coming together.
* Convocation: This implies a gathering of people for a specific purpose.
* Congregation: This often refers to a gathering of people for religious purposes.
Informal:
* Get-together: This is a casual term for a social gathering.
* Meet-up: This emphasizes the coming together of people.
* Coming together: This is a simple and straightforward way to describe people coming together.
* Catch-up: This suggests that people are getting together to talk about what has been happening in their lives.
Specific to family or friends:
* Family gathering: This is a common term for a gathering of family members.
* Class reunion: This refers to a gathering of people who were in the same class at school.
* Alumni reunion: This refers to a gathering of graduates of a school or university.
The best synonym for "reunion" will depend on the specific context in which it is being used.