Spooling
Buffering output to disk so a slow peripheral can be fed at its own pace without stalling the processor.
The name is a back-formation: Simultaneous Peripheral Operations On-Line. The problem was concrete on 1960s mainframes — a line printer or card reader ran a thousand times slower than the processor, and letting a program write to it directly left the expensive machine idle. The fix was to write the job to a fast disk or tape and let a separate program feed the printer from there. IBM's HASP on the System/360 is the classic example. The same idea survives every time a document is "sent to the printer" and control returns at once: it lands in a queue first. Spooling was also an early step toward multi-user operation.