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 (Meng).
usage
Often shortened to 内蒙古 or just 内蒙 (Nèiměng) in everyday speech. Note 蒙 in 蒙古 is ménggǔ with a falling-rising tone, distinct from the verb 蒙 (to cover, méng).