Attribute Domain is a set of range of values allowed in an attribute Eg: rooms in hotel (range 1-300) Age (1-99) Married (yes-no) Nationality (Sri Lankan, Indian, American, British) for an instance if we take Nationality attribute and classify a domain using the above nationalities. the user can only choose his nationality only from those above data (Sri Lankan, Indian, American, British) {{expert}} 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 can't 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.