Yermak or Ermak (both: yĕrmäk´), d. 1584?, Russian conqueror of Siberia; his name also occurs as Yermak Timofeyevich. The leader of a band of independent Russian Cossacks, he spent his early career plundering the czar's ships on the Volga and later entered the service of a merchant family, the Stroganovs. They sent Yermak on an expedition to protect their lands in W Siberia from attack by local tribes. Advancing in river boats, Yermak and his band crossed the Urals and with the superior force of firearms conquered (1582) the capital of the Tatar khanate of Sibir; he placed the conquered territory under the protection of Czar Ivan IV and asked him for aid. Yermak was killed in an encounter with the Tatars, and his troops were forced to retreat. However, Russian troops retook the territory in 1586.