Definition: A line of code placed in an ad or on a web page that helps track the visitor’s actions, such as registrations or purchases. A web beacon is often invisible because it’s only 1 x 1 pixel in size and has no color. Also known as web bug, 1 by 1 GIF, invisible GIF or tracker GIF.
FirstClickSEO Commentary: This is a tracking device used for referrals, commissions, and e-commerce environments.