If a set of digital signals are plesiochronous, then their transitions occur at almost the same rate, with a limited variation. The variation is due to different signals in the set being derived from different reference clocks. However, the transitions do not necessarily occur at the same time, unlike in a synchronous signal set.
Inter-networking between continents can be seen as plesiochronous because the systems all operate at slightly different rates.
The term is from the greek plesos meaning "close" and chronous, meaning "time".