Final answer:
A horizontal form that is supported at only one end is known as a cantilever, distinct from arches, trusses, and beams.
Step-by-step explanation:
The structure that is a horizontal form supported at only one end is known as a cantilever. This type of structure extends horizontally from a vertical support, such as a wall, and is not supported at the other end. Unlike an arch, which is a compression form that can span a large area by resolving forces into compressive stresses, or a truss or beam, which are typically supported at both ends, the cantilever is distinctive in its singular support design. One key application of cantilever construction is in balconies, where the horizontal surface is unsupported on its projecting end, creating an overhang.