Operating System Command Line Interface.
Jargon for the BBC Micro's command interpreter, a low-tech command shell used back on this venerable machine. It was also a BBC BASIC keyword, which passed the supplied string parameter to the interpreter.
When the Beeb grew up into the Archimedes and later machines, this terminology was kept, and survived on as a software interrupt (in Acorn parlance SWI), "OS_CLI".