Formal:
* Environment: This is the most common synonym and emphasizes the surrounding conditions.
* Context: This emphasizes the setting and circumstances that surround an event or situation.
* Setting: This emphasizes the physical location or background.
* Ambiance: This emphasizes the mood or atmosphere of a place.
Informal:
* Scene: This is a more casual term for the environment or surroundings.
* Circle: This refers to a group of people with shared interests or backgrounds.
* World: This implies a broader sense of environment and its influence.
* Culture: This emphasizes the social customs and values of a particular group.
The best synonym for "milieu" depends on the specific context and the nuance you want to convey.