A pterygium is a winglike triangular membrane. Although a pterygium can be anywhere, including behind the knee, it commonly refers to a winglet of the conjunctiva (a thin clear moist membrane that coats the inner surfaces of the eyelids and the outer surface of the eye).
The section of the conjunctiva that coats the inner aspect of the eyelids is called the palpebral conjunctiva while that covering the outer surface of the eye is called the ocular or bulbar conjunctiva. Inflammation of the conjunctiva is called conjunctivitis and leads to pink eye.
This pterygium may extend across the white of the eye toward the inner corner of the eye. It is caused by prolonged exposure of the eyes to wind and weather, or can be an inherited disorder caused by a single gene.