Definition
A nosewheel that is not mechanically linked to the rudder pedals and is free to swivel left or right on its own. Directional control on the ground is achieved by using differential braking on the main wheels, and sometimes by rudder effectiveness from propeller slipstream, rather than by steering the nosewheel directly.
Plain English
The front wheel can spin freely from side to side. The pilot turns the airplane by braking one main wheel harder than the other, not by steering the nose wheel.
Context Anchor
Seen in tricycle landing gear discussions, especially when comparing airplanes with steerable nosewheels to airplanes that use free-swiveling nosewheels.
Derivation
From 'caster,' the small swiveling wheel found on shopping carts and office chairs. Like those wheels, a castering nosewheel pivots freely to follow whichever direction the aircraft is being pushed, rather than being steered directly.
Why Pilots Care
It makes taxiing and ground turns simpler by letting the pilot use rudder or differential braking without needing a separate steering mechanism.
Analogy
Like a shopping cart wheel: it points wherever the cart is pushed. You don't turn the wheel itself -- you steer the cart, and the wheel follows.
Intuition Check
Do not assume a castering nosewheel is a steerable nosewheel. If it casters, it swivels freely; the pilot controls direction by rudder effect and separate use of the left and right brakes.
Example Sentence 1
Because the SR22 has a castering nosewheel, the pilot used light taps of the right brake to line up on the taxiway centerline.
Example Sentence 2
After landing, the castering nosewheel allowed the airplane to follow the taxiway centerline without additional steering effort.