Definitions

Taxiing. Movement of an aircraft on the surface of an aerodrome under its own power, excluding take-off and landing.

Air-taxiing. Movement of a helicopter/VTOL above the surface of an aerodrome, normally in ground effect and at a ground speed normally less than 37 km/h (20 kt).
Note. The actual height may vary, and some helicopters may require air-taxiing above 8 m (25 ft) AGL to reduce ground effect turbulence or provide clearance for cargo slingloads.

Source: ICAO Doc 4444 PANS-ATM

SkyBrary photo by Thomas Young