正好找到一份FAA的NOTICE：《Operations Over Uninhabited Terrain Areas》
The requirements of § 121.353 were implemented to increase the chance of survival and rescue for airplane occupants in the event of an accident or emergency in areas of uninhabited terrain. The Preamble to the Final Rule that amended § 121.353 in 1995 (60 FR 65832) did not attempt to change requirements for flying over uninhabited terrain, nor did it attempt to clarify or amend the definition of “uninhabited terrain.” The Preamble stated that defining uninhabited terrain is “not necessary …[s]ince implementation is on a case-by-case basis.” Currently, no legal interpretation that speaks to the definition of “uninhabited terrain” exists and none are expected.
妈的原来FAA也说没定义。顺手找到文中提到的1995（60 FR 65832）：
In addition, this rulemaking does not define ‘‘uninhabited terrain.’’ When the predecessor regulation to § 121.353 was proposed in CAB draft release 58–24 in 1960, ‘‘uninhabited terrain’’ was defined as ‘‘flights for long distances over frigid or tropical land areas for which the Director finds such equipment to be necessary for search and rescue operations because of the character of the terrain to be flown over.’’ When the rule was adopted, the wording was changed to provide the Administrator more flexibility in identifying uninhabited areas.
Since implementation is on a case-by-case basis through operations specifications, it was determined that the proposed wording was not necessary. This provision has been in effect for over 30 years without any problem about the meaning of ‘‘uninhabited areas.’’
其实我看不太懂1995（60 FR 65832）到底说的是什么，看样子是在修订规章时的流水账。