Established on 1 May 1947 - before the founding of the PRC - making it the oldest of China's five 自治区 (autonomous regions). 内 (inner) distinguishes it from 外蒙古 (Outer Mongolia), the independent country of Mongolia since 1921. Short name 蒙.
usage
Often shortened to 内蒙古 or just 内蒙 in everyday speech. Note that 蒙 in 蒙古 is read měng (third tone), distinct from the verb 蒙 (to cover).