Pterygium: A wing-like triangular membrane. Although a pterygium can be anywhere, including behind the knee, it commonly refers to a winglet of the conjunctiva. 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 it can be an inherited disorder caused by a single gene. A pterygium can be removed with surgical procedures including laser treatment.