retronym
    n 1: a word introduced because an existing term has become
         inadequate; "Nobody ever heard of analog clocks until
         digital clocks became common, so `analog clock' is a
         retronym"

retronym

    A term invented to distinguish a subclass of things
   from new members of the superclass, where the distinction was
   previously not necessary, since the old subclass had been all
   there was of the superclass.

   For example, the retronyms "snail mail" and "paper mail"
   were coined by those for who "mail" was likely to mean
   electronic mail.

   While the English language in general has a few retronyms
   ("whole milk", "snow skiing", "acoustic guitar"), hacker
   jargon is necessarily (at points capriciously) rich in
   retronyms, e.g. plaintext, natural language, impact
   printer, eyeball search, biological virus.

   [More examples?]

   (2001-02-25)