The ability to send a
message from one
computer to another over the
Internet, with that message popping up in a
message box on the target computer's display.
America Online did much to popularize the concept of instant messaging. One of the most popular instant-messaging utilities today is ICQ, also owned by AOL.
Jabber is an open-source instant messaging protocol using streaming XML as its underlying data format. It is compatible with other instant messaging systems like ICQ or AIM.