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

embedded system

    Hardware and software which forms a component of
   some larger system and which is expected to function without
   human intervention.

   A typical embedded system consists of a single-board
   microcomputer with software in ROM, which starts running
   some special purpose application program as soon as it is
   turned on and will not stop until it is turned off (if ever).

   An embedded system may include some kind of operating system
   but often it will be simple enough to be written as a single
   program.  It will not usually have any of the normal
   peripherals such as a keyboard, monitor, serial connections,
   mass storage, etc. or any kind of user interface software
   unless these are required by the overall system of which it is
   a part.  Often it must provide real-time response.

   Usenet newsgroup: news:comp.arch.embedded.

   (1995-04-12)