The Collaborative International Dictionary of English v.0.48:

Visor \Vis"or\, n. [OE. visere, F. visi[`e]re, fr. OF. vis. See
   Visage, Vision.] [Written also visar, visard,
   vizard, and vizor.]
   1. A part of a helmet, arranged so as to lift or open, and so
      show the face. The openings for seeing and breathing are
      generally in it.
      [1913 Webster]

   2. A mask used to disfigure or disguise. "My very visor began
      to assume life." --Shak.
      [1913 Webster]

            My weaker government since, makes you pull off the
            visor.                                --Sir P.
                                                  Sidney.
      [1913 Webster]

   3. The fore piece of a cap, projecting over, and protecting
      the eyes.
      [1913 Webster]

WordNet (r) 3.0 (2006):

visor
    n 1: a piece of armor plate (with eye slits) fixed or hinged to
         a medieval helmet to protect the face [syn: visor,
         vizor]
    2: a brim that projects to the front to shade the eyes; "he
       pulled down the bill of his cap and trudged ahead" [syn:
       bill, peak, eyeshade, visor, vizor]

Moby Thesaurus II by Grady Ward, 1.0:

24 Moby Thesaurus words for "visor":
   bill, camouflage, dark glasses, disguise, domino, eyeshade,
   eyeshield, false colors, false face, false front, goggles,
   incognito, mask, masque, masquerade, mummery, peak,
   protective coloration, shades, smoke screen, smoked glasses,
   sunglasses, vizard, vizard mask