Documented in Volume 1 of the UNIX Programmers Manual.
....is a 4.2bsd command
% talk
a souped-up, full-screen version of write. This is a vast
improvement over prior software. Your party's terminal must be 'writeable'
(see the example for 'finger -s' to determine this...)
% rwho | grep ferris
ferris bass:ttyi3 Aug 6 08:40 :02
% talk bass:ferris
This example demonstrates how to target and then talk to another user
on another UNIX machine on the local network. The rwho generates a list from
which grep lists one line: the user you want to talk to. Since ferris is
on another machine (this command is being run from cod), talk needs to
know with which machine to interact.