Caution, vague. It includes Mongolia, but what else the term includes has varied over time. Alexander von Humboldt (1769-1859), a German naturalist, helped popularize the term and included large parts of Russia, Iran and Afghanistan. Others disputed his methodology. A more modern definition would include Mongolia, Kazakhstan, Kyrgyzstan, Tajikistan, Turkmenistan and Uzbekistan.
Reference: AAJA Handbook to Covering Asian America