a multiframe is a frame on which a cannibalistic operation is applied.
some examples…
a frame multiplied by an infinite number, it’s a circular frame (useful to slice the timeslots into half-rate)
if it is a frame multiplied by another frame, the you call is a square frame. That’s the basis of the Edge (one frame has 8 edges, very sharp ones).
there are other explanations, but not as satisfying… have a look :
http://www.pulsewan.com/data101/gsm_basics.htm