Plain English Meaning
An airplane that has its small third wheel at the back, under the tail, instead of at the front under the nose. On the ground, the nose points up and the tail sits low.
Full Definition
An airplane with conventional landing gear, meaning two main wheels under the forward part of the fuselage and a small wheel under the tail. The aircraft sits in a nose-high attitude on the ground, with the tail low and the propeller pointed upward.
Context Anchor
Seen in flight instructor endorsements when a pilot is being trained and signed off to fly tail wheel airplanes.