General Synonyms:
* Style
* Trend
* Mode
* Vogue
* Craze
* Fad
* Manner
* Custom
* Practice
More specific synonyms depending on context:
* For clothing: apparel, attire, garb, dress, outfit
* For a particular style: look, aesthetic, vibe
* For a trend: wave, movement, phenomenon
The best synonym for "fashion" will depend on the specific context in which you are using it.