Plesiochronous - Signals which are arbitrarily close in frequency to some
defined precision.  They are not sourced from the same clock and so, over
the long term, will be skewed from each other.  Their relative closeness
of frequency allows a switch to cross connect, switch, or in some way
processs them. That same inaccuracy of timing will force a switch, over
time, to repeat or delete frames (called frame slips) in order to handle
buffer underflow or overflow. 

See also: Asynchronous, Isochronous, Synchronous