What is the term for a lens consisting of two cylinders of equal but opposite powers?

A lens consisting of two cylinders of equal but opposite powers is referred to as a cross cylinder. This type of lens is specifically designed to correct astigmatism. It has two cylindrical surfaces that are oriented perpendicularly to one another, allowing for the differentiation of the two principal meridians of an astigmatic eye. The opposite powers help in sharpening the vision by compensating for the unequal curvature of the cornea or lens in astigmatism.

In terms of the other options, a concave lens is used to diverge light rays and is primarily used for myopia (nearsightedness), a cylindrical lens has a single cylinder that can correct for astigmatism but does not have opposing powers, and a plano lens is a flat lens that does not alter the direction of light rays and is generally used for protective purposes or as a carrier lens in optical systems. These distinctions highlight why cross cylinder is the appropriate term for the specific configuration described in the question.