The part of an Internet URL (Universal Resource Locator) selected and registered by an individual, business or organization to represent their web presence. It consists of at least two parts that are separated by “dots.” The last part or suffix indicates the type of site, such as “.edu” for schools, “.gov” for government, and “.com” for commercial business.

Source:
http://www.nielsenmedia.com/glossary/
http://www.arbitron.com/downloads/terms_brochure.pdf