Plain English Meaning
The short cross-wise leg you fly just before turning onto final, sitting at 90 degrees to the runway near its approach end.
Full Definition
A flight path in the airport traffic pattern flown at right angles to the landing runway off its approach end, connecting the downwind leg to the final approach leg.
Context Anchor
Used during airport traffic pattern flying and radio position reports, especially when a pilot says they are turning base or on base.
Derivation
From the idea of the traffic pattern as a rectangular circuit. 'Base' refers to the side of that nearest the — the leg that forms the base of the before the final .