The word chat is typically associated with "Real-time" online conferencing [L1 ]. Tcl has been the basis for MANY implementations of chatting software, including

FPX: As for the difference between chat and IM -- I'd say that a chat is something that two or more people sign up (log in) for (like IRC), whereas an IM is spontaneous. According to that definition, only the first IM message would be instant; after that (when the recipient accepts the conversation), IM becomes a chat.

see also instant messaging.