A visor, also known as an eyeshade, was a type of transparent surface that protected the eyes.
A specially designed visor was used to shield the eyes of humanoids who may look upon Medusans in order to prevent the insanity that would otherwise result from their hideous appearance. It proved to be especially effective for Vulcans such as Commander Spock. Miranda Jones wore one as well to hide the fact that she was blind and could not see Ambassador Kollos. (TOS: "Is There in Truth No Beauty?")
Certain poker players wore transparent eyeshades. Lieutenant Commander Data regularly wore one of these during his poker games with the senior staff of the USS Enterprise-D. (TNG: "Cause and Effect", "Descent", "All Good Things...")