Definition of a Domain
A domain is a data type defined with explicit constraints. A data type is a well-defined set of values. For example, the data type INTEGER represents all the possible integer numbers, while an INTEGER column defined with an explicit CHECK constraint condition such as “count BETWEEN 25 AND 50” defines a domain on the set of numbers having the INTEGER data type. More unambiguous data types are usually application‑specific and you can define them using the various facilities available to you for defining user‑defined types and their associated constructs such as methods, orderings, and so on. See SQL Data Definition Language and SQL External Routine Programming for more details about user‑defined data types.