Web Bug

A tiny image, invisible to a user, placed on Web pages in such a way to enable third parties to track use of Web servers and collect information about the user, including IP address, host name, browser type and version, operating system name and version, and cookies.
SOURCE: SP 800-28

Malicious code, invisible to a user, placed on Web sites in such a way that it allows third parties to track use of Web servers and collect information about the user, including IP address, host name, browser type and version, operating system name and version, and Web browser cookie.
SOURCE: CNSSI-4009