Definition 1 of 2
Plain English Meaning
Driving the aircraft along the ground at an airport using its own engine power, getting from where it is parked out to the runway, or back from the runway to parking. For helicopters, it also covers low, slow movement just above the ground when the situation calls for it.
Full Definition
The controlled movement of an aircraft on the surface of an airport under its own power, excluding takeoff and landing. Taxi includes movement on taxiways, ramps, and other airport surfaces, and for helicopters operating without ground contact at airports with movement areas it also includes air taxi and operations.