A logical grouping, used to organize networked computers. On the Internet, the highest-level domain is the country; sub-domains, listed to the left and separated by dots, work through an increasingly specific hierarchy in order to identify a unique location, known as the 'domain name.' Reading from right to left is analogous to reading from the bottom to the top line of a postal address.
From: Communications

A logical grouping, used to organize networked computers. On the Internet, the highest-level domain is the country; sub-domains, listed to the left and separated by dots, work through an increasingly specific hierarchy in order to identify a unique location, known as the 'domain name.' Reading from right to left is analogous to reading from the bottom to the top line of a postal address.
From: Internet

A sub-set of internet addresses. Domains are hierarchical, and lower-level domains often refer to particular web sites within a top-level domain. The most significant part of the address comes at the end - typical top-level domains are .com, .edu, .gov, .org (which sub-divide addresses into areas of use). There are also various geographic top-level domains (e.g. .ar, .ca, .fr, .ro etc.) referring to particular countries.
The relevance to search engine terminology is that web sites which have their own domain name (e.g. http://www.nativetongues.com) will often achieve better positioning than web sites which exist as a sub-directory of another organisation's domain (e.g. http://ourworld.compuserve.com/homepages/tijana/).
From: Search Engines