A complainer; one who speaks out when there are problems. Often used pejoratively. / Of several problems, the most apparent or the most urgent.

(automotive, originally late 1950s and into 1960s) A wheel for a motor vehicle, made from an alloy of magnesium, typically lighter and offering better heat conduction than steel wheels.

In control of a vehicle; driving. / (idiomatic) In control of the situation; in charge.

(mechanics) A mechanism that translates a continuous rotation into an intermittent rotary motion.

A device locked to the wheel of a motor car to prevent it from being driven; normally used to enforce payment of a traffic violation fine

(rail transport) On a steam locomotive, an unpowered wheel or axle located behind the driving wheels.

a revolving crib set in the wall of a church or convent, allowing an abandoned baby to be left anonymously and safely.

On a steam locomotive, an unpowered wheel located in front of the driving wheels.

(rail transport) On a steam locomotive, a powered wheel driven by the locomotive's pistons or turbine.

The combination button/scroll wheel selection device of some iPods.