It is stated as error in focussing of light by eye resulting in dimness of vision.
800 million to 2.3 billion in the globe have refractive errors.

Myopia: Optics too powerful for length of eye ball due to more curvature or larger axial length of eye ball. Also called as Near sightedness and corrected using concave lens.

Hypermetropia: Optics too weak for length of eye ball due to not enough curvature or lesser axial length of eye ball. Also called as Hyeropia or Far sightedness and corrected using convex lens.

Presbyopia: An age related condition where the flexibility of lens declines and causing trouble to read. Corrected using Progressive, Bifocals or Reading glasses.

Astigmatism: A person with astigmatic refractive error sees lines of a particular orientation less clearly than lines at right angles to them. This defect can be corrected by cylindrical lens.

Treatment: Refractive errors can be corrected using Spectacles, Contact lens and LASIK Surgery.