Plain English Meaning
A racetrack-shaped path the pilot flies in circles around a chosen point in the sky, used to wait when ATC isn't ready to let the aircraft continue.
Full Definition
A predetermined racetrack-shaped flight path used to keep an aircraft within a defined block of airspace while awaiting further clearance. A standard holding pattern consists of two 180-degree right turns connected by two straight legs, flown relative to a fixed reference point called the holding fix. Legs are timed (typically one minute below 14,000 feet, one and a half minutes above) or defined by distance when using DME or GPS.
Context Anchor
You will see on and hear them in when an aircraft must wait before continuing along a or .