AIM 5-6-5 maps the FAA's designated mountainous areas. If you are within the boundaries, whether you are actually flying in mountains, over plateaus, mesas, or flat desert, you are flying in a mountainous area. It's basically the FAA's way of deciding what obstacle clearance is required in certain areas. e.g. In mountainous areas you are given 2000' clearance for MEAs, 1000' in non-mountainous areas.
If you use Jeppesen approach plates then there is a map in the back of the packet of the United States called "Air Defense Identification Zones and Mountainous Terrain Areas" which digrams all the areas designated as mountainous or non-mountainous. I once heard from someone that mountainous areas are areas in which the terrain extends above 5000 MSL but I have never seen this doucumented so I don' t know.
Pronunciation: 'maun-t&n-&s, 'maunt-n&s
Date: 14th century
1 : containing many mountains
2 : resembling a mountain : HUGE
- moun·tain·ous·ly adverb
- moun·tain·ous·ness noun
Pronunciation: t&-'rAn also te-
Etymology: French, land, ground, from (assumed) Vulgar Latin terranum, alteration of Latin terrenum, from neuter of terrenus of earth -- more at TERRENE
1 a (1) : a geographical area (2) : a piece of land : GROUND b : the physical features of a tract of land
2 : TERRANE 1
3 a : a field of knowledge or interest : TERRITORY b : ENVIRONMENT, MILIEU