1. Simultaneous Peripheral Operation On-Line - To send files to some device or program (a `spooler') that queues them up and does something useful with them later. From Linux Guide @FirstLinux

2. vi. [from early IBM `Simultaneous Peripheral Operation On-Line', but is widely thought to be a backronym] To send files to some device or program (a `spooler') that queues them up and does something useful with them later. Without qualification, the spooler is the `print spooler' controlling output of jobs to a printer; but the term has been used in connection with other peripherals (especially plotters and graphics devices) and occasionally even for input devices. See also demon. From Jargon Dictionary