General:
* Area: This is a broad term that encompasses a wide range of topics or activities.
* Field: This word suggests a specific area of study or expertise.
* Realm: This term implies a particular sphere of influence or activity.
* Sphere: Similar to realm, this word suggests a distinct area of activity or interest.
* Scope: This word refers to the extent or range of something.
* Territory: This word suggests a specific area of control or influence.
In specific contexts:
* Website: Website address, URL
* Mathematics: Set
* Science: Kingdom, Phylum, Class, Order, Family, Genus, Species
* Biology: Ecosystem, Biome, Niche
More informal options:
* Territory
* Stomping grounds
* Area of expertise
* Speciality
For a more precise replacement, please provide me with more context about how you are using the word "domain."