In
telecommunications, a
plesiochronous system is one where different parts of the system are almost, but not quite, perfectly
synchronised. According to
ITU-T standards, a pair of signals are plesiochronous if their significant instants occur at nominally the same rate, with any variation in rate being constrained within specified limits. A sender and receiver operate
plesiosynchronously if they operate at the same nominal frequency but may have a slight frequency mismatch, which leads to a drifting phase. The mismatch between the two systems' clocks is known as the
plesiochronous difference.