Yermak Timofeyevich (; born between 1532 and 1542 – August 5 or 6, 1584) was a Cossack who led the Russian conquest of Siberia in the reign of Ivan the Terrible."Timofeyevich" is his patronimic, not the last name, so in references he must be looked up as "Yermak" or "Yermak Timofeyevich"