In my experience Outer Mongolia is often used more or less synonymous with the four Khalkha aimags at least post 1757 If I remember correctly this was also the Qing Dynasty s terminology But I don t now how the territorial status of the Khalkha aimags changed between Galdan s death and 1757 except that one aimag was added in 1725 and of course it was the nobles who submitted not the land So if you feel like clarifying that part of the article please do