Web beacon

A web beacon is a graphic image (such as a pixel tag or clear GIF) that is placed on a Web page or in an e-mail message to monitor user activity (such as whether the Web page or e-mail message is read or clicked). They are often invisible to the user and may be very small in size. They are also used on Web pages for alignment purposes.

It is difficult limit the use of Web beacons since there is no easy way to determine whether they are being used. They also may be loaded from a different Web server than the rest of the page.