A domain is defined as the set of all unique values permitted for an attribute.

The domain of a database attribute is the set of all allowable values for that attribute.

Example:

- A field for gender may have the domain {male, female, unknown} where those three values are the only permitted entries in that column.
- A domain of date is the set of all possible valid dates.
- A domain of integer is all possible whole numbers.
- A domain of day-of-week is Monday, Tuesday … Sunday.