Plain English Meaning
The last segment of the approach to landing, where the aircraft is lined up with the runway and descending toward it.
Full Definition
In instrument flight, that part of an instrument approach procedure in which alignment and descent for landing are accomplished, beginning at the final approach fix or point and ending at the missed approach point or runway. In visual flight, the flight path of an aircraft on the last leg of the traffic pattern, aligned with the extended runway centerline and descending toward the landing runway.