Pinguecula refers to benign degenerative conjunctival lesions, usually found on the nasal side of each eye.
Published figures of prevalence range from 11 - 75% (prevalence depends on age and geographical location). Most people over 70 show some signs.
The exact cause has not been determined, but It's believed to be associated with excess exposure to UV light - and is more prevalent in hot dry sunny regions of the world. It may also be associated with dry air, dust, and the wearing of contact lenses
More info College of Optometrists