U

Unique click:
A unique click is a single click by a single user. When unique clicks are measured, it is an aggregate number of how many times that URL was clicked by individual users. Related to the unique users metric for web servers.

Unsolicited Commercial E-mail (UCE):
Also known as Spam. Commercial e-mail sent without the recipient's express permission.

Unsubscribe:
To remove oneself from an email list, either via an emailed command to the list server or by filling in a Web form.

Universal Resource Locator (URL):
The address that defines the route to a website, page or any other resource on the Internet. URLs are typed into a Web browser to access Web pages and files, and URLs are embedded within the pages themselves as hypertext links. The URL contains the protocol prefix, port number, domain name, subdirectory names and file name.