.uk is the Internet country code top-level domain (ccTLD) for the United Kingdom. As of April 2010, it is the fourth most popular top-level domain worldwide (after .com, .de and .net), with over 8.6 million registrations.
The use of .uk rather than .gb for the top-level domain is due to its pre-existing use in the (now obsolete) JANET Name Registration Scheme in which the order of address components were reversed. .uk was made available in DNS to simplify the translation to and from these addresses. There were plans for eventual transition to .gb to accord with correct usage, but this never occurred; the use of .uk is now entrenched as it is impossible to register a .gb domain.
Unusually, the '.co.uk' second level domain within '.uk' is more widely used than for most other countries where the ccTLD has primacy. This is a result of specific rules set by Nominet, the UK's naming body, prohibiting direct use of the ccTLD. The single exception to this rule is the Nominet website itself.