Plain English Meaning

These are the altitudes a pilot must fly when operating in the clouds or in low visibility under instrument rules. They are chosen so the aircraft stays clear of terrain, obstacles, and other traffic, even when the pilot cannot see outside.

Full Definition

The published or assigned altitudes a pilot is required to fly while operating under Instrument Flight Rules. These include minimum en route altitudes, minimum obstruction clearance altitudes, minimum vectoring altitudes, and altitudes assigned by ATC, all of which are designed to guarantee terrain and obstacle clearance, navigation signal reception, and safe from other traffic.