Official Aviation Definition
In a standard standard airport traffic pattern, the base leg is the flight path flown at right angles (perpendicular) to the landing runway centerline. It connects the downwind leg to the final approach leg.
In Simple Words
The second-to-last turn in the traffic pattern around a runway before turning onto final approach to land.
Real-World Flight Example
The tower cleared the pilot to turn left base for runway 24 behind the departing Boeing 737.