Which feature is characteristic of a cross cylinder lens?

A cross cylinder lens is defined by its unique structure, which incorporates two cylindrical surfaces oriented at right angles to each other. This characteristic design allows for the correction of astigmatism by addressing two distinct meridians of the eye simultaneously. Each cylinder has its own power and axis, which is why the answer highlights the two cylinders as the defining feature.

In comparison to the other choices: the presence of two prisms is not applicable since prisms work by bending light rather than correcting for astigmatism, and a lens that incorporates one sphere refers to spherical lenses which are used for myopia or hyperopia but do not correct astigmatism. Variable power typically describes progressive lenses, which provide different powers for different viewing distances, whereas cross cylinder lenses have fixed powers based on their cylindrical design. Thus, the presence of two cylinders is what distinctly characterizes a cross cylinder lens.