Plain English Meaning
A landing gear setup with two wheels in the middle of the aircraft and one wheel at the nose, so the airplane sits flat on the ground.
Full Definition
A landing gear arrangement consisting of two main wheels positioned aft of the aircraft's center of gravity and a single nosewheel forward of it, giving the aircraft a level (or near-level) attitude on the ground.
Context Anchor
Seen in taxiing, takeoff, landing, and aircraft handling discussions, especially when comparing nosewheel airplanes with tailwheel airplanes.
Derivation
From the bicycle/tricycle family of words: 'tri-' (Latin/Greek for three) plus 'cycle' (wheel). The name simply describes the three-wheel layout, but the key feature is where those wheels are placed — one at the nose, two under the or .