General:
* Class
* Group
* Type
* Kind
* Sort
* Genre
* Division
* Section
* Department
* Branch
* Variety
* Classification
* Grouping
* Subset
More specific:
* Genre (for books, movies, etc.)
* Domain (for knowledge areas)
* Field (for areas of study or work)
* Sector (for industry)
* Taxonomy (for scientific classification)
The best synonym to use will depend on the specific situation and the intended meaning.