The domain - an admissible potential limited subset of values of this type. For example, the domain NAMES is defined on base type of lines of symbols, but the number of its values can include only those lines which can represent names (in particular, for possibility of representation of Russian names such lines cannot begin with a soft or hard sign and cannot be longer 20 symbols). One domain can include values from several columns united, besides identical type of data, also logically. If two values undertake from the same domain, it is possible to execute comparison of these two values.
Simpler definition of the domain - this admissible potential set of values of one type.
Domain name - this name serving for identification of areas - units of an administrative autonomy on the Internet - as a part of higher on hierarchy of such area. Each of such areas is called as the domain. The general namespace of the Internet functions thanks to DNS - system of domain names. Domain names give the chance to addressing of Internet knots and the network resources located on them (websites, servers of e-mail, other services) in a form convenient for the person.
The full domain name consists of a direct name of the domain and further names of all domains into which it enters, divided by points. For example, the full name "ru.wikipedia.org" designates the domain of the third level of "ru" which enters into the domain of the second level of "wikipedia" which enters into the domain of the top level of "org" which enters into the anonymous root domain "." (point). In ordinary speech under domain quite often understand full domain name.