Plain English Meaning
The set rules for flying a racetrack-shaped waiting pattern over a fixed point during the cruise portion of an IFR flight, used when ATC needs you to stay in one area for a while.
Full Definition
The standardized methods pilots use to fly a holding pattern along an en route segment of an IFR flight, typically at an intersection, fix, or navaid, when ATC requires the aircraft to remain in a specific area of airspace. En route holding involves entering and flying the published or ATC-assigned pattern using the standard right-turn racetrack (or if specified), at the assigned and within the limits prescribed for the holding altitude.