An instant messenger system is one that allows people on the Internet to find and communicate with one another in real time (unlike e-mail).
InstantMessenger services and protocols
Would it be correct to say that the Unix "talk" facility - which (at least in later versions?) could be used across the Internet between compatible Unixen) - was an early form of instant messaging? -- ApoorvaMuralidhara
I think that's close to correct. Of course talk was only for the wimps who could not cope with a real Unix tool such as write. -- DavidVincent