verb Of a medium, substance, object, etc.: to deflect the course of (light rays), esp. when they enter the medium, etc., at an oblique angle; to cause refraction of (light, other electromagnetic radiation, or sound or other wave phenomena).
verb To mediate; to alter; to distort.
verb To cause (light) to change direction as a result of entering a different medium.
verb To measure, and often also to correct with lenses, the refractive error of (an eye) or the eyes of (a person).