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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 to 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.