Which of the following terms describes the primary correction power of a prescription lens?

The term that describes the primary correction power of a prescription lens is sphere power. Sphere power refers to the strength of the lens that corrects for refractive errors in the eye, specifically myopia (nearsightedness) or hyperopia (farsightedness). It is measured in diopters and indicates whether the lens is convex (plus power for hyperopia) or concave (minus power for myopia).

Cylinder power, on the other hand, refers to the additional power that corrects astigmatism, which is a different type of refractive error. The axis indicates the orientation of astigmatism correction but does not represent the overall correction power. Addition refers to the extra magnifying power added for reading in multifocal lenses, typically prescribed for presbyopia, and is also not indicative of the primary refractive correction. Therefore, when considering the term that specifically defines the primary lens correction for refractive errors, sphere power is the accurate term.