* a name that is entered into a computer (e.g. as part of a Web site or other URL, or an e-mail address) and then looked up in the global Domain Name System which informs the computer of the IP address(es) with that name.
* the product that registrars provide to their customers.
* a name looked up in the DNS for other purposes.

They are sometimes colloquially (and incorrectly) referred to by marketers as "Web addresses".

* The authoritative definition is that given in the RFCs that define the DNS.

Domain names are hostnames that provide rememberable names to stand in for numeric IP addresses. They allow for any service to move to a different location in the topology of the Internet (or another internet), which would then have a different IP address.

Each string of letters, digits and hyphens between the dots is called a label in the parlance of the domain name system (DNS). Valid labels are subject to certain rules, which have relaxed over the course of time. Originally labels must start with a letter, and end with a letter or digit; any intervening characters may be letters, digits, or hyphens. Labels must be between 1 and 63 characters long (inclusive). Letters are ASCII A–Z and a–z; domain names are compared case-insensitively. Later it became permissible for labels to commence with a digit (but not for domain names to be entirely numeric), and for labels to contain internal underscores, but support for such domain names is uneven. These are the rules imposed by the way names are looked up ("resolved") by DNS. Some top level domains (see below) impose more rules, such as a longer minimum length, on some labels. Fully qualified names (FQDNs) are sometimes written with a final dot.

Translating numeric addresses to alphabetical ones, domain names allow Internet users to localize and visit Web sites. Additionally since more than one IP address can be assigned to a domain name, and more than one domain name assigned to an IP address, one server can have multiple roles, and one role can be spread among multiple servers. One IP address can even be assigned to several servers, such as with anycast and hijacked IP space.

The following examples illustrates the difference between a URL (Uniform Resource Locator) and a domain name:

URL: http://www.example.com/
Domain name: www.example.com

As a general rule, the IP address and the server name are interchangeable. For most Internet services, the server will not have any way to know which was used. However, the explosion of interest in the Web means that there are far more Web sites than servers. To accommodate this, the hypertext transfer protocol (HTTP) specifies that the client tells the server which name is being used. This way, one server with one IP address can provide different sites for different domain names. This feature goes under the name virtual hosting and is commonly used by Web hosts.

For example, the server at 192.0.34.166 handles all of the following sites:

www.example.com
www.example.net
www.example.org