- noun a system that gives people on the Internet the ability to communicate in real time
- noun a feature that lets a user type in and exchange messages with one or more other people connected via the Internet. Each of the users in the group runs special software that tells them when a friend or colleague has connected to the Internet and is available to receive messages. Any message that he or she types in is then sent instantly to the other user.
- noun software which allows computer users to send short messages in real time which appear on another user’s screen