Which aspect of a lens prescription indicates where the cylinder power is oriented?

The aspect of a lens prescription that indicates where the cylinder power is oriented is the axis. The axis measurement provides the degree of the angle in which the cylindrical component of the lens is positioned to correct astigmatism. It is given in degrees, with values ranging from 1 to 180, and corresponds to the orientation of the meridian of the eye that requires the cylinder power for proper focus.

Sphere refers to the lens power used to correct nearsightedness or farsightedness but does not provide any information about the orientation of the cylinder power. Addition is associated with multifocal lenses, indicating the additional power needed for near vision but is unrelated to the cylindrical correction. Prism, on the other hand, refers to the degree of prismatic effect prescribed to correct binocular vision problems, again not relevant to the orientation of cylinder power. Thus, the axis is essential for understanding how the lens should be positioned to effectively address the patient's astigmatism.