An accretionary wedge is a large, wedge-shaped mass of sediment and oceanic crust that accumulates at convergent plate boundaries, where one plate is subducted beneath another. The sediments are scraped off the subducting plate and piled onto the overriding plate, forming complex structures of folded and faulted rocks. Accretionary wedges are key in understanding the tectonic evolution of convergent margins and the processes involved in mountain building.
Reference: https://www.usgs.gov/media/images/accretionary-wedge.