The Mohorovičić discontinuity ( MOH-hə-ROH-vih-chitch; Croatian: [moxorôʋiːtʃitɕ]) – usually called the Moho discontinuity, Moho boundary, or just Moho – is the boundary between the crust and the mantle of Earth. It is defined by the distinct change in velocity of seismic waves as they pass through changing densities of rock.