The Collaborative International Dictionary of English v.0.48:

Visible \Vis"i*ble\, a. [L. visibilis, fr. videre, visum, to
   see: cf. F. visible. See Vision.]
   1. Perceivable by the eye; capable of being seen;
      perceptible; in view; as, a visible star; the least spot
      is visible on white paper.
      [1913 Webster]

            Maker of heaven and earth, and of all things visible
            and invisible.                        --Bk. of Com.
                                                  Prayer.
      [1913 Webster]

            Virtue made visible in outward grace. --Young.
      [1913 Webster]

   2. Noticeable; apparent; open; conspicuous. --Shak.
      [1913 Webster]

            The factions at court were greater, or more visible,
            than before.                          --Clarendon.
      [1913 Webster]

   Visible church (Theol.), the apparent church of Christ on
      earth; the whole body of professed believers in Christ, as
      contradistinguished from the invisible, or real, church,
      consisting of sanctified persons.

   Visible horizon. Same as Apparent horizon, under
      Apparent.
      [1913 Webster] -- Vis"i*ble*ness, n. -- Vis"i*bly,
      adv.
      [1913 Webster]

WordNet (r) 3.0 (2006):

visible
    adj 1: capable of being seen; or open to easy view; "a visible
           object"; "visible stars"; "mountains visible in the
           distance"; "a visible change of expression"; "visible
           files" [syn: visible, seeable] [ant: invisible,
           unseeable]
    2: obvious to the eye; "a visible change of expression"
    3: present and easily available; "the cash on hand is adequate
       for current needs"; "emergency police were on hand in case of
       trouble"; "a visible supply"; "visible resources"

Moby Thesaurus II by Grady Ward, 1.0:

72 Moby Thesaurus words for "visible":
   apparent, appearing, beholdable, clear, clear as crystal,
   conspicuous, crystal-clear, detectable, discernible, disclosed,
   discoverable, distinct, evident, explicit, exposed,
   exposed to view, express, eyeball, hanging out, identifiable,
   in evidence, in full view, in plain sight, in view, indisputable,
   indubitable, insight, manifest, naked, noticeable, observable,
   obvious, ocular, open, open to view, open-and-shut, ophthalmic,
   optic, ostensible, outcropping, outward, palpable, patent,
   perceivable, perceptible, perspicuous, plain, plain as day,
   prominent, recognizable, revealed, seeable, seeing, seeming, seen,
   self-evident, self-explaining, self-explanatory, showing,
   superficial, surface, tangible, to be seen, unclouded, unconcealed,
   undisguised, unhidden, unmistakable, viewable, visual,
   well-defined, witnessable