Clear or slightly colored plastic lenses worn directly on the eyeball to correct vision problems, held in place over the cornea by a thin layer of tears.

The concept of corrective lenses that sit directly on the eyeball was developed in the ninth century, but it was not until the late 20th century that manufacturing and grinding techniques for contact lenses were perfected. Originally contact lenses were made of glass, then later of hard plastic, and still later of flexible, highly oxygen-permeable soft plastic. Today, the majority of lenses worn are of two basic types: rigid gas permeable (so-called "hard" lenses) and soft lenses. Hard lenses are smaller and more durable, but require a longer period of adjustment for the wearer. Hard contact lenses can correct some vision problems, like astigmatism, that the soft lenses cannot. The soft lenses are slightly larger than hard lenses. They...