The Free On-line Dictionary of Computing (30 December 2018):

Simultaneous Peripheral Operation On-Line

    (SPOOL) Accessing peripheral
   devices with the help of an off-line tape drive.  The term
   was derived by IBM for use with the IBM 360 operating
   systems.

   In the early days of computing (early 1960s), before
   multitasking was invented, computers (e.g. IBM 704) could
   run only one job at a time.  As peripheral devices such as
   printers or card readers were much slower than the CPU,
   devoting the computer (the only computer in many cases) to
   controlling such devices was impractical.

   To free the CPU for useful work, the output was sent to a
   magnetic tape drive, which was much faster than a printer
   and much cheaper than a computer.  After the job was finished
   the tape was removed from the tape drive attached to the
   computer and mounted on a tape drive connected to a printer
   (such as the IBM 1403).  The printer could then print the
   data without holding up the computer.  Similarly, instead of
   inputting the program from the card reader it was first copied
   to a tape and the tape was read by the computer.

   (1999-01-12)