Plain English Meaning
Maps pilots use when they are flying by instruments and following ATC routings instead of looking out the window for landmarks. They show the radio-based highways in the sky, the altitudes you must fly, and the procedures for arriving and departing at airports.
Full Definition
Aeronautical charts designed for use under Instrument Flight Rules, depicting airways, navigation aids, intersections, minimum altitudes, controlled airspace boundaries, and instrument procedures rather than visual ground references. The two main types are Enroute Low/High Altitude and Publications (, , and charts).