Blindness was a medical condition affecting the sufferer's eyes, with a large number of possible causes. It caused difficulties with vision, ranging from minor impairment to total loss.
Related conditions were day blindness and night blindness, in which the sufferer was blinded in overly bright or overly dark conditions; color blindness, which caused a blindness to certain colors; and distorted vision such as with astigmatism.
Geordi La Forge was born completely blind due to an "incurable optical nerve defect." (TNG episodes: "Encounter at Farpoint", "Conundrum"; TNG video game: The Transinium Challenge) As a child he was given a VISOR device that allowed him to see. (TNG episode: "Hero Worship")