Plain English Meaning
These are the maps pilots use when flying by instruments between airports. They show the highways in the sky, the radio aids that mark them, the lowest safe altitudes, and who to talk to along the way.
Full Definition
Aeronautical charts designed for navigation under instrument flight rules (IFR) during the enroute phase of flight. They depict airways, navigation aids, intersections, minimum altitudes, controlled airspace boundaries, communication frequencies, and reporting points needed to fly between airports without visual reference to the ground.