1. the class of entities or events that constitutes the subject matter of a science or other discipline.

2. a field of mastery, dominance, or endeavor.

3. in biological taxonomy, the highest category used in some classification systems, comprising one or more kingdoms. Three domains are recognized: Archaea (archaebacteria), Bacteria, and Eukarya (including animals, plants, fungi, and protists).

4. in set theory, the set of elements over which a function is defined.