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

microtape

    /mi:'kroh-tayp/ Occasionally used to mean
   a DECtape, as opposed to a macrotape.

   Apparently the term "microtape" was actually the official term
   used within DEC for these tapes until someone coined the word
   "DECtape", which, of course, sounded sexier to the
   marketroids.  Another version of the story holds that
   someone discovered a conflict with another company's
   "microtape" trademark.

   [Jargon File]

   (1995-03-16)