write (Unix)
Unix's oldest way to talk to another user: type lines, and they appear in the middle of their terminal.
The command was already present in the earliest Unix releases of the early 1970s. write user opened a one-way channel to that user's terminal, and every line you typed was printed there — right over whatever they were doing, be it an editor session or a compile. To reply, the recipient had to start their own write in the other direction. Convention borrowed from radio: end your turn with "o", end the conversation with "oo".
Anyone who wanted peace and quiet ran mesg n, which revoked write access to their terminal. The sibling command wall writes to everyone logged in at once, and is still what you see when a server is about to go down. talk replaced write for actual conversations in 1983.