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):

visibly
    adv 1: in a visible manner; "he was visibly upset" [ant:
           invisibly]
    2: so as to be visible; "the sign was visibly displayed"

Moby Thesaurus II by Grady Ward, 1.0:

97 Moby Thesaurus words for "visibly":
   actually, after a fashion, apparently, appreciably, as seen,
   assuredly, at a glance, at any rate, at best, at first sight,
   at least, at most, at sight, at the least, at the most,
   at the outside, at worst, by eyeball, by sight, certainly, clearly,
   comparatively, conspicuously, decidedly, definitely, demonstrably,
   detectably, discernibly, distinctly, evidently, explicitly,
   expressly, fairly, for a certainty, for real, glaringly,
   in a manner, in a way, in all conscience, in part, in some measure,
   incompletely, indeed, indubitably, leastwise, manifestly, markedly,
   merely, mildly, moderately, modestly, not comprehensively,
   not exhaustively, noticeably, observably, obviously, only,
   palpably, part, partially, partly, patently, perceivably,
   perceptibly, plainly, positively, prima facie, pro tanto,
   prominently, pronouncedly, purely, quite, really, recognizably,
   relatively, seeably, sensibly, seriously, simply, so far, somewhat,
   staringly, starkly, thus far, to a degree, to some degree,
   tolerably, truly, unambiguously, unconcealedly, undeniably,
   undisguisedly, unmistakably, verily, visually, with clarity,
   without doubt