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I don't know for sure but my understanding is same as Hamrad's. I know that in reality what we call a fifthwheel is not the fifthwheel, it's a 'fifthwheel trailer' and the hitch in the bed of the truck is the 'fifthwheel'. I may be wrong but that's my understanding.
Here are some definitions from a google search on 'fifthwheel definition'.
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fifth wheel
n.
1. A wheel or portion of a wheel placed horizontally over the forward axle of a carriage to provide support and stability during turns.
2. A similar device over the rear axle or axles of a tractor or pickup truck, serving as a coupling for a semitrailer.
3. An additional wheel carried on a four-wheeled vehicle as a spare.
4. An extra and unnecessary person or thing.
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fifth wheel - a steering bearing that enables the front axle of a horse-drawn wagon to rotate
bearing - a rotating support placed between moving parts to allow them to move easily
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fifth wheel
n 1: an extra car wheel and tire [syn: spare]
2: a steering bearing that enables the front axle of a
horse-drawn wagon to rotate
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