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. Pterygia are associated with eyelid inflammation, called Blepharitis. Growth over the corneal centre, which leads to dramatic reduction of vision. Induced anterior corneal scarring, which often remains after surgical removal.A pterygium of the eye grows very slowly. Usually it takes several years or decades to progress. Surgical removal
Indications for surgery, in order of decreasing importance:

Growth over the corneal centre. Reduced vision due to corneal distortion. Documented growth. Symptoms of discomfort. Cosmesis.Surgery is usually performed under local anaesthetic with light sedation as day surgery. The pterygium is stripped carefully off the surface of the eye.
This occurs in three stages, an initial attachment to the skin, followed by the creeping over the skin searching for a suitable penetration site, often a hair follicle, and finally penetration of the skin into the epidermis using cytolytic secretions from the cercarial post-acetabular, then pre-acetabular glands. On penetration, the head of the cercaria transforms into an endoparasitic larva, the schistosomule. Each schistosomule spends a few days in the skin and then enters the circulation starting at the dermal lymphatics and venules. Here, they feed on blood, regurgitating the haem as hemozoin. The schistosomule migrates to the lungs (5–7 days post-penetration) and then moves via circulation through the left side of the heart to the hepatoportal circulation (>15 days) where, if it meets a partner of the opposite sex, it develops into a sexually mature adult and the pair migrate to the mesenteric veins. Such pairings are monogamous.Male schistosomes undergo normal maturation and morphological development in the presence or absence of a female, although behavioural, physiological and antigenic differences between males from single-sex, as opposed to bisex, infections have been reported. On the other hand, female schistosomes do not mature without a male. Female schistosomes from single-sex infections are underdeveloped and exhibit an immature reproductive system. Although the maturation of the female worm seems to be dependent on the presence of the mature male, the stimuli for female growth and for reproductive development seem to be independent from each other.