instant messagingn. The transmission of an electronic message over a computer network using software that immediately displays the message in a window on the screen of the recipient.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.instant messaging n (Computer Science) computing the online facility that allows the instant exchange of written messages between two or more people using different computers or mobile phones. Abbreviation: IM Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014instant messagingA program that allows users to communicate with each other instantaneously using typed messages.Dictionary of Unfamiliar Words by Diagram Group Copyright © 2008 by Diagram Visual Information Limited