Simultaneous Peripheral Operation On-Line. Though it probably originated in the thought that you have a "spool" of magnetic tape. Nowadays we don't use the term much except in the context of printing: we "spool" a print job to disk and it goes to the printer when the printer is ready and jobs ahead of it are done. The "print spooler" handles all this for us.
Originally, though, spooling was for card readers. Before this, cards were first read and written to tape, and the tape was then mounted for the cpu to read. This was much more efficient than having the cpu read the cards during program execution, and was the crude beginning of multi-processing systems.
Way back then, you might have also heard the term "staging" used to describe reading an entire tape to disk before processing it.
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