Plain English Meaning
An airplane that sits level on the ground with one wheel at the front and two wheels further back under the wings or fuselage.
Full Definition
An airplane with a landing gear arrangement consisting of two main wheels positioned aft of the center of gravity and a single nosewheel forward of the center of gravity. This configuration keeps the fuselage roughly level on the ground and places the nosewheel under the engine and forward fuselage.
Context Anchor
You will see this term when discussing takeoffs, landings, taxiing, and the after-landing roll, especially when comparing nosewheel airplanes with tailwheel airplanes.