Plain English Meaning
An airplane that rests on two big wheels at the front and a small wheel at the back, so its nose points up when it's on the ground.
Full Definition
An airplane whose landing gear consists of two main wheels forward of the center of gravity and a small wheel at the tail, causing the airplane to sit on the ground in a nose-high attitude.
Context Anchor
Seen in taxiing, takeoff, and landing discussions where the airplane’s ground handling and forward visibility are important.
Derivation
Named simply for the location of the small third wheel — at the tail, rather than under the nose. Sometimes called a 'taildragger' for the same reason.