Taxiing
Taxiing, also sometimes written "taxying", is the movement of an aircraft on the ground, under its own power, in contrast to towing or push-back where the aircraft is moved by a tug. The aircraft usually moves on wheels, but the term also includes aircraft with skis or floats (for water-based travel).
An airplane uses taxiways to taxi from one place on an airport to another; for example, when moving from a terminal to the runway. The term "taxiing" is not used for the accelerating run along a runway prior to takeoff, or the decelerating run immediately after landing.
Propulsion
The thrust to propel the aircraft forward comes from its propellers or jet engines. Reverse thrust for backing up can be generated by thrust reversers such as on the Boeing C-17 Globemaster III, or reversible pitch propellers such as on the Lockheed C-130 Hercules. Most aircraft, however, are not designed to back up on their own and must be pushed back either by hand or by using an aircraft tug.
At low power settings, combustion aircraft engines operate at lower efficiency than at cruise power settings. A typical A320 spends an average of 3.5 hours a day taxiing, using 150 US gallons (570 L) of fuel. Hybrid electrically driven nosegear are under development to allow high use aircraft to shut down the engines during taxi operations.