talk mode n.
A feature supported by Unix, ITS, and some other OSes that
allows two or more logged-in users to set up a real-time on-line
conversation. It combines the immediacy of talking with all the
precision (and verbosity) that written language entails. It is
difficult to communicate inflection, though conventions have
arisen for some of these (see the section on writing style in the
Prependices for details).
Talk mode has a special set of jargon words, used to save
typing, which are not used orally. Some of these are identical to
(and probably derived from) Morse-code jargon used by ham-radio
amateurs since the 1920s.