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]

Moby Thesaurus II by Grady Ward, 1.0:

63 Moby Thesaurus words for "invisible":
   atomic, behind the curtain, behind the scenes, camouflaged,
   concealed, corpuscular, covered, disguised, embryonic, evanescent,
   germinal, granular, hidden, impalpable, imperceptible,
   imponderable, inappreciable, indiscernible, infinitesimal,
   insensible, intangible, latent, masked, microcosmic, microscopic,
   molecular, out of sight, secret, sightless, subatomic, submerged,
   tenuous, thin, ultramicroscopic, unapparent, unappreciable,
   unbeheld, unbeholdable, undetected, undiscernible, undisclosed,
   undiscovered, undivulged, unexplained, unexplored, unexposed,
   unnoticed, unobservable, unobserved, unperceivable, unperceived,
   unrealized, unrevealed, unseeable, unseen, unsolved, unspied,
   untraced, untracked, unviewed, unwitnessed, veiled, viewless