instant messaging (noun)
A method of real-time communication over the Internet in which a sender types a message to one or more recipients and the recipient immediately receives the message in a chat area.

IM (noun)
A message sent in real time through a network to a private chat area.

IM (noun)
A method of real-time communication over the Internet in which a sender types a message to one or more recipients and the recipient immediately receives the message in a chat area.