Web beacon

A web beacon is a technique used on web pages and email to unobtrusively (usually invisibly) allow checking that a user has accessed some content. Web beacons are typically used by third parties to monitor the activity of users at a website for the purpose of web analytics or page tagging. They can also be used for email tracking. When implemented using JavaScript, they may be called JavaScript tags. Web beacons are unseen HTML elements that track a webpage's views.