Definitions for "Web bug"
A small graphic inserted in an email or web page that alerts a spammer when a message is read or previewed.
see Web beacon.
Also called a web beacon, pixel tag, clear GIF (graphics interchange format) or invisible GIF. A way for an HTML email message's sender to determine if and when the message was opened and read.
Keywords:  camouflaged, otherwise, clear
a clear, camouflaged or otherwise
Keywords:  compression