Domain

noun

1. A geographic area owned or controlled by a single person or organization.

   "The king ruled his domain harshly."

2. A field or sphere of activity, influence or expertise.

   "Dealing with complaints isn't really my domain: get in touch with customer services."

3. A group of related items, topics, or subjects.
4. The set of all possible mathematical entities (points) where a given function is defined.
5. The set of input (argument) values for which a function is defined.