From Wiktionary, the free dictionary
web bug (plural web bugs)
- (Internet, marketing) A small, usually transparent image added by an advertiser to a webpage (to track its popularity) or email message (to track when it is read).
- Synonyms: tag, tracking bug, tracking pixel, page tag, pixel tag, web beacon, web tag
2002, John R. Levine, Ray Everett-Church, Greg Stebben, Internet Privacy For Dummies, Wiley, →ISBN:When a Web bug is programmed into a Web page, its code looks similar to the code for just about any graphic image appearing on that page.
2011, Andrew McStay, The Mood of Information: A Critique of Online Behavioural Advertising, Bloomsbury, →ISBN:Google had a web bug on 92 of the top 100 sites, and on 88 per cent of the total domains reported in the data set of almost 400,000 unique domains.
web beacon on Wikipedia.Wikipedia 