A cataract is a very common condition of the lens (a structure located inside the eye which, along with the cornea, focuses light on the retina for clear vision). A cataract is a clouding or opacity...
It all depends on what type of "cataract" is being discussed.
Niagara Falls could be a cataract. So could the gradual opacification that occurs in the lens of the eye as a natural result of aging....
There is only one "type" of cataract, which is a clouding of the eye's lens. Though this is true, cataracts can be divided into classifications based on cause (age-related, traumatic, congenital,...