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What is Pterygium and explain in detail?
Pterygium (plural pterygia or pterygiums) refers to any wing-like triangular membrane occurring in the neck, eyes, knees, elbows, ankles or digits.The term comes from the Greek word pterygion meaning "wing".

Types
Popliteal pterygium syndrome, a congenital condition affecting the face, limbs, or genitalia but named after the wing-like structural anomaly behind the knee
Pterygium (eye) or surfers eye, a growth on the cornea of the eye
Pterygium colli or webbed neck,  a congenital skin fold of the neck down to the shoulders
Pterygium inversum unguis or ventral pterygium, adherence of the distal portion of the nailbed to the ventral surface of the nail plate
Pterygium unguis or dorsal pterygium, scarring between the proximal nail fold and matrix

Pterygium of the eye
A pterygium reduces the vision in several ways:

Distortion of the corneal optics.  This begins usually when the pterygium is greater than 2mm from the corneal edge (limbus).
Disruption of the tear film.  The tear film is the first lens in the eye