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

plesiochronous

    Nearly synchronised, a term describing a
   communication system where transmitted signals have the same
   nominal digital rate but are synchronised on different clocks.

   According to ITU-T standards, corresponding signals are
   plesiochronous if their significant instants occur at
   nominally the same rate, with any variation in rate being
   constrained within specified limits.

   [Pronunciation?  /ples'ee-oh-kroh'nus/?]

   (1995-03-02)