engine

   1.  A  piece of hardware that encapsulates some function but can't be
   used  without  some  kind  of {front end}. Today we have, especially,
   print engine: the guts of a laser printer.

   2. An analogous piece of software; notionally, one that does a lot of
   noisy crunching, such as a database engine.

   The  hacker  senses  of  engine  are  actually close to its original,
   pre-Industrial-Revolution   sense  of  a  skill,  clever  device,  or
   instrument  (the  word is cognate to `ingenuity'). This sense had not
   been    completely    eclipsed   by   the   modern   connotation   of
   power-transducing machinery in Charles Babbage's time, which explains
   why he named the stored-program computer that he designed in 1844 the
   Analytical Engine.

[glossary]