The human crystalline lens, which is clear and transparent is part of the focusing mechanism of the eye. With age, the lens becomes cloudy and opaque, thereby hampering normal vision. Any opacity in the crystalline lens, which leads to decreased vision, is cataract..
The early cataractous lens bends light irregularly, so the images formed are blurred and unclear. When opaque, the cataract may cut off light nearly in total, causing blindness (reversible).
Ageing is the most common cause. With age, proteins of the eye lens get altered and opacified leading to cataract. Other causes include congenital and developmental anomalies, trauma, inflammation of the eye, metabolic diseases like Diabetes, Hypothyroidism etc. .