WebBug

typically an invisible image (e.g. a 1x1 transparent image) (sometimes a bit of JavaScript) placed in a page to help a system measure Web Traffic.

often used in HTML broadcast EMail to track how many people read a message (when the person reads the message, the image gets requested from the server).

Sometimes a WebBug is delivered by a 3rd-party service, as part of their Web Traffic- or WebAd-analysis service. This means any Web Cookie associated with the WebBug is from the domain of the 3rd party, which means that 3rd party can aggregate a single user's data across multiple clients.