The Collaborative International Dictionary of English v.0.48:

Invisible \In*vis"i*ble\, n.
   1. An invisible person or thing; specifically, God, the
      Supreme Being.
      [1913 Webster]

   2. A Rosicrucian; -- so called because avoiding declaration
      of his craft. [Obs.]
      [1913 Webster]

   3. (Eccl. Hist.) One of those (as in the 16th century) who
      denied the visibility of the church. --Shipley.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Invisible \In*vis"i*ble\, a. [F. invisible, L. invisibilis. See
   In- not, and Visible.]
   1. Incapable of being seen; not perceptible by vision; not
      visible. Specifically:
      (a) Not visible due to an inherent property, such as lack
          of color; as, the invisible air; invisible ink;
      (b) hidden from view; out of sight;
      (c) Not perceptible due to lack of light;
      (d) Too small or too distant to be perceived; as, people
          on the ground invisible at cruising altitude.
          [1913 Webster +PJC]

                To us invisible, or dimly seen
                In these thy lowest works.        --Milton.
          [1913 Webster]

   2. Hidden from the public; as, invisible transactions.
      [PJC]

   3. imperceptible to the mind; as, differences invisible to
      most observers.
      [PJC]

   Invisible bird (Zool.), a small, shy singing bird
      (Myadestes sibilons), of St. Vincent Islands.

   Invisible green, a very dark shade of green, approaching to
      black, and liable to be mistaken for it.
      [1913 Webster]

WordNet (r) 3.0 (2006):

invisible
    adj 1: impossible or nearly impossible to see; imperceptible by
           the eye; "the invisible man"; "invisible rays"; "an
           invisible hinge"; "invisible mending" [syn: invisible,
           unseeable] [ant: seeable, visible]
    2: not prominent or readily noticeable; "he pushed the string
       through an inconspicuous hole"; "the invisible man" [syn:
       inconspicuous, invisible] [ant: conspicuous]