Just in Case Someone Asks You What an Octothorpe Is
An octothorpe is the "#" symbol, which you might call "pound sign," "number sign," or "square." Evidently, in the 1960s, some Bell Lab engineers wanted to give the symbol a special name and started using octothorpe. As you probably already guessed, the name didn't really stick.
Thanks to Adrian Lurssen and Mark Nemeth for expanding my horizons. - K