A jockey wheel is a retractable adjustable-height wheel used on the front of trailers with either a single axle (two running wheels) or more close-coupled axles at or near the centre of gravity whereby without additional support, the trailer would not remain 'level'. The jockey wheel is situated close to the towing hitch and has a built-in screw jack to enable the trailer nose to be lifted over the tow ball of a car or other powered vehicle. The screw jack can then be used to gently lower the trailer nose onto the tow ball. Once securely attached to the towing vehicle, the jockey wheel's jacking action is fully retracted for stowage. The jockey wheel is also unclamped and lifted as far as possible to give the greatest ground clearance before reclamping prior to a journey being made.

Jockey wheels incorporate a castor

Caster

A caster is an undriven, single, double, or compound wheel that is designed to be mounted to the bottom of a larger object so as to enable that object to be easily moved...

action, permitting travel in any direction while manhandling a trailer while it is not attached to a vehicle.