Final answer:
The official definition of a semi-trailer does not include it having a front axle; semi-trailers lack front axles and depend on a towing vehicle for mobility. They can be unhitched and have no motive power of their own.
Step-by-step explanation:
The official definition of a semi-trailer does not include the following aspect: it has a front axle. By definition, a semi-trailer is a trailer without a front axle. A large proportion of its weight is supported by a semi-truck, tractor, or a detachable front axle assembly known as a dolly. A semi-trailer is designed to be towed by a motor vehicle and does indeed have no motive power of its own. Therefore, it is dependent on the towing vehicle for movement. An important characteristic of a semi-trailer is that it can indeed be unhitched from the towing vehicle.
To clarify the concepts not related to semi-trailers, but mentioned in the additional context: a wheel on a wheelbarrow indeed has no fulcrum. However, in the case of a wheelbarrow, the center of the axle serves as the point about which the wheel rotates, which is different from the way in which a load is attached to an axle in other contexts such as that of a semi-trailer.