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.
In general, plesiochronous systems behave similarly to synchronous systems, except they must employ some means in order to cope with "sync slips", which will happen at intervals due to the plesiochronous nature of the system. The most common example of a plesiochronous system design is the plesiochronous digital hierarchy networking standard. Wikipedia, Plesiochronous
See Also
Discord
Discordant
Enharmonic
Graduation
Power of Beat Harmonics