The term plesiochronous is derived from the Greek plesio, meaning near, and chronos, time, and refers to the fact that plesiochronous systems run in a state where different parts of the system are almost, but not quite perfectly, synchronised.
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. In general, plesiochronous systems behave similarly to synchronous systems, except that they must have some means 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.
The modern tendency in systems engineering is towards using systems that are either fundamentally asynchronous (such as Ethernet), or fundamentally synchronous (such as SDH), and layering these where necessary, rather than using a mixture between the two in a single technology.