General:
* Outfit
* Attire
* Apparel
* Dress
* Garb
* Habiliment
* Regalia
* Uniform
* Ensemble
For specific occasions:
* Fancy dress: For parties or events where people dress up in imaginative costumes.
* Disguise: When the costume is used to conceal someone's identity.
* Masquerade: For a ball or party where people wear masks.
* Regalia: For formal occasions or ceremonial events.
* Uniform: For specific occupations, organizations, or groups.
Other related words:
* Clothing
* Dress-up
* Outfit
* Ensemble
* Appearance
* Gearing (for specific activities like sports)
The best synonym for "costume" will depend on the specific situation and the tone you want to convey.