Glasses (also called spectacles) are lenses held in front of the eyes by frames made of metal or plastic in order to improve defective eyesight. Contact lenses work similarly but sit directly on the cornea of the wearer.
By the 23rd century advances in medical science had made the wearing of glasses very rare. Root normally wears contact lenses but when scrambling to respond to a red alert wears glasses instead. (TOSS #19 )