In computing, the attribute domain is the set of values allowed in an attribute.

For example: Rooms in hotel (1-300) Age (1-99) Married (yes or no) Nationality (Sri Lankan, Indian, American, or British)

For the relational model it is a requirement that each part of a tuple be atomic. The consequence is that each value in the tuple must be of some basic type, like a string or an integer. For the elementary type to be atomic it cannot be broken into more pieces. Alas, the domain is an elementary type, and attribute domain the domain a given attribute belongs to an abstraction belonging to or characteristic of an entity.