Mohorovičić discontinuity, often called Moho, is the boundary between Earth's crust and mantle - see the image below. It was discovered by Croatian seismologist Andrija Mohorovičić. He discovered discontinuity in velocity of seismic wave propagation when studying an earthquake near Zagreb on October 8, 1909. His main focus of study were P-waves (presure waves) and S-waves (secondary waves). S-waves always comes after P-waves in seismograph because S-waves cannot travel through Earth's mantle which lies beyond Moho. This is one of most significant discoveries in seismology because of which Mohorovičić is considered founder of modern seismology.
It's worth mentioning that Mohorovičić discontinuity exists not only on our planet, but on other planets as well, probably on all planets with a molten core.