Source : Free On-Line Dictionary of Computing

spool file
     
        Any file to which data is {spool}ed to await the next stage of
        processing.  Especially used in circumstances where spooling
        the data copes with a mismatch between speeds in two devices
        or pieces of software.  For example, when you send mail under
        Unix, it's typically copied to a spool file to await a
        transport {demon}'s attentions.
     
        [{Jargon File}]