Beacon

A web beacon is a small object, such as a 1 pixel gif, embedded in markup, used to communicate information back to the web server or to third-party servers. Beacons are generally included to provide information about the user for statistical purposes. Beacons are often included within third-party scripts for collecting user data, performance metrics and error reporting.

There is a W3C Draft Beacon Specification to standardize the beacon as an interface to asynchronously transfer HTTP data from User Agent to a web server prior to page load without negative performance impact.