Official Aviation Definition
A holding pattern is a predetermined maneuver designed to keep an aircraft within a specified airspace while awaiting further ATC clearance. Standard holds utilize right-hand turns in a racetrack shape, with 1-minute inbound legs below 14,000 feet MSL.
In Simple Words
A racetrack-shaped path an airplane flies in circles high in the sky when an airport is too busy for landing.
Real-World Flight Example
Due to thunderstorm activity over Chicago O'Hare, incoming flights were placed in holding patterns over entry fixes.