A simple definition of a database domain is the data type used by a column in a database. This data type can be a built-in type (such as an integer or a string) or a custom type that defines constraints on the data.
DBMS – Attribute and Domain
When you enter data into an online form of any kind, whether it’s your name and email address or a job application, a database stores your input behind the scenes. That database evaluates your input based on a set of criteria.
For example, if you enter a zip code, the database expects to find five numbers (or five numbers followed by a hyphen and then four numbers for a full U.S. zip code). If you enter your name in a zip code field, the database gives you an error.
That's because the database tests your input against the domain defined for the zip code field. A domain is essentially a data type that can contain optional constraints.
