Plain English Meaning
A recommended route, published by the FAA, that pilots should file when flying IFR between certain busy airports. Filing it usually means you get the clearance you asked for, instead of being given a different route by ATC.
Full Definition
A published IFR route established between busy terminal areas to standardize traffic flow, reduce controller workload, and minimize route changes during clearance delivery. Preferred IFR routes are listed in the Chart Supplement and are designed to be filed by pilots when flying between specific city pairs, often with separate routings for low and .