wall (Unix)
"Write all": the command that prints a message straight onto every logged-in user's terminal at once.
wall is the loud sibling of write and has been part of Unix since the early 1970s. You typed your text, hit Ctrl-D, and every terminal on the machine got it in the middle of whatever was going on — an editor session, a compile, a talk conversation. The most frequent sender was shutdown, announcing at regular intervals that the system was going down in five minutes.
Users who had closed their terminal with mesg n were spared ordinary users' walls, but not root's. On shared university machines it was one of the few ways for the sysadmin to reach everyone at once, and one of the few ways for a bored student to become unpopular with everyone at once. The command survives in modern Linux, and systemd still sends its shutdown warnings that way.