Lake Peipus (pī´pəs), Estonian Peipsi Järv, Rus. Chudskoye Ozero, c.1,390 sq mi (3,600 sq km), dividing Estonia from the W Pskov region, Russia. Its southern section is known as Lake Pskov. Lake Peipus, which is navigable, empties through the Narva River into the Gulf of Finland. On the frozen strait between Lake Peipus and Lake Pskov, Alexander Nevsky defeated the Livonian Knights in 1242.