The Moho is the boundary between the crust and the mantle in the earth. This is a depth where seismic waves change velocity and there is also a change in chemical composition. Also termed the Mohorovicic' discontinuity after the Croatian seismologist Andrija Mohorovicic' (1857-1936) who discovered it. The boundary is between 25 and 60 km deep beneath the continents and between 5 and 8 km deep beneath the ocean floor.
(Image courtesy of Massey University)