What is the definition of mountainous area?
(I'm assuming you're asking about the USA.)

14 CFR 91.177(a)(2)(i) says (emphasis mine):


  In the case of operations over an area designated as a mountainous
  area in part 95 of this chapter, an altitude of 2,000 feet above the
  highest obstacle within a horizontal distance of 4 nautical miles from
  the course to be flown; or [...]


If you then go to 14 CFR 95, you'll see that it defines mountainous areas by latitude and longitude. It's entirely useless for practical purposes, but at least it's precise :-)

As for the MOCA, I can't find anything to support the different definitions that you mentioned. 95.1(e) simply says:


  (e) The MOCA assures obstruction clearance on an ATS route, ATS route
  segment, or other direct route, and adequate reception of VOR
  navigation signals within 22 nautical miles of a VOR station used to
  define the route.


The AIM and PC/G support that; there's no other definition that I could find.