(2)
Other than mountainous areas, 1000 feet above the highest obstacle within a horizontal distance of 4 nautical miles from the course to be flown; or

(3)
As otherwise authorized by the Administrator or assigned by Air Traffic Control. (See Minimum Narrate IFR Altitude, Minimum Obstruction Clearance Altitude, Minimum Crossing Altitude, Minimum Safe Altitude, Minimum Vectoring Altitude) (Refer to Federal Air Regulations Part 91)

(SPEC/Pilot-Controller Glossary & FAR 91.177)

MINIMUM NAVIGATION PERFORMANCE SPECIFICATIONS AIRSPACE (IFIM) - A concept adopted by ICAO with the objective of ensuring safe separation of aircraft and maximum benefit from required navigation equipment.
MINIMUM OBSTRUCTION CLEARANCE ALTITUDE/MOCA (P.CG, JCS) - The lowest published altitude in effect between radio fixes on VOR airways, off-airway routes, or route segments which meets obstacle clearance requirements for the entire route segment and which assures acceptable navigation signal coverage only within 25 statue miles (22 nautical miles) of a VOR. (Refer to Federal Air Regulations Part 91 and 95)
MINIMUM RECEPTION ALTITUDE/MRA (P/CG) - The lowest altitude at which an intersection can be determined. (Refer to Federal Air Regulations Part 95)
MINIMUM RECEPTION ALTITUDE (JCS) - The lowest altitude required to receive adequate signals to determine specific VHF omnirange/tactical air navigation fixes.
MINIMUM SAFE ALTITUDE/MSA (P/CG) -
(1) The minimum altitude specified in Federal Air Regulations Part 91, for various aircraft operations.
(2) Altitudes depicted on approach charts which
provide at least 1000 feet of obstacle clearance for emergency use within a specified distance from the navigation facility upon which a procedure is predicated. These altitudes will be identified as Minimum Sector Altitude or Emergency Safe Altitude and are established as follows:
(a)
MINIMUM SECTOR ALTITUDES - Altitudes depicted on approach charts which provide at least 1,000 feet of obstacle clearance within a 25-mile radius of the navigation facility upon which the procedure is predicated. Sectors depicted on approach charts must be at least 90 degrees in scope. These altitudes are for emergency use only and do not necessarily assure acceptable navigational signal coverage.

(b)
EMERGENCY SAFE ALTITUDES - Altitudes depicted on approach charts which provide at least 1,000 feet of obstacle clearance in non-mountainous areas and 2,000 feet of obstacle clearance in designated mountainous areas within a 100-mile radius of the navigational facility upon which the procedure is predicated and normally used only in military procedures. These altitudes are identified on published procedures as "Emergency Safe Altitudes".

MINIMUM SAFE ALTITUDE (JCS, NATO) - The altitude below which it is hazardous to fly, owing to presence of high ground or other obstacles.
MINIMUM SAFE ALTITUDE WARNING (MSAW) - A function of the Automated Radar Terminal Systems III computer that aids the controller by alerting him when a tracked Mode C equipped aircraft is below or is predicted by the computer to go below a predetermined Minimum Safe Altitude.