An ocean trench is a deep, elongated depression in the ocean floor, typically formed at convergent plate boundaries where one tectonic plate is subducted beneath another. Ocean trenches are the deepest parts of the ocean and are significant for studying subduction zones, the recycling of oceanic crust, and the dynamics of plate tectonics.
Reference: Stern, R. J. (2002). “Subduction Zones.” Reviews of Geophysics, 40(4), 3-1-3-38.