The top-level domain name in an Internet address, such as .com, .org and .edu. See Internet domain name and extension.
Domain extensions, or suffixes, are categories of Internet domain names, and serve to describe the type of company, organization, country or another category.
The technical term used for domain extensions is top-level-domain (or TLD). There are many categories from which to choose, and some extensions are more popular than others.
Be aware that for the majority of domain extensions, some exceptions apply, the category the domain claims to represent is not enforced. A domain name with the extension .co.uk may be representing a UK based business, but you cannot rely on this as anybody is allowed to register a .co.uk domain name.