Plain English Meaning
An airplane that rests on two big wheels up front and one small wheel under the tail, so it sits with its nose pointed up while on the ground.
Full Definition
Airplanes whose landing gear consists of two main wheels forward of the center of gravity and a single small wheel at the tail. Because the center of gravity is behind the main wheels, these airplanes sit nose-high on the ground and require active rudder control during taxi, takeoff, and landing to keep the nose tracking straight.
Context Anchor
Used in discussions of taxiing, takeoff, landing rollout, and ground loops in airplanes with conventional landing gear.