The accretionary wedge is like the accretionary prism and the definition of that is : a mass of sedimentary material scraped off a region of oceanic crust during subduction and piled up at the edge of the overriding plate (source : dictionary) and the formation is when two plates crash and one slides under the other one I believe and sand ad sediment go into the little open space between them that give you a wedge. Hope I helped.
