Plesiochronous difference - In telecommunications, if two digital signals are plesiochronous, their transitions occur at nearly the same rate, with any variation being within preset limits. EXAMPLE: If two networks are working together their clocks may be using two different Primary Reference Clocks (PRC). These clocks are extremely accurate (+ or - 1 in 1011) and employ caesium atomic standards. That said, there are differences between PRCs. This is known as a plesiochronous difference.