The Collaborative International Dictionary of English v.0.48:

Observable \Ob*serv"a*ble\, a. [L. observabilis: cf. F.
   observable.]
   1. Capable of being observed; discernible; noticeable. --Sir.
      T. Browne.
      [1913 Webster]

            The difference is sufficiently observable.
                                                  --Southey.
      [1913 Webster]

   2. Worthy of being observed; important enough to be noted or
      celebrated; as, an observable anniversary.
      [PJC]

   3. Noteworthy; remarkable.
      [PJC] -- Ob*serv"a*ble*ness, n. -- Ob*serv"a*bly, adv.
      [1913 Webster]

WordNet (r) 3.0 (2006):

observable
    adj 1: capable of being seen or noticed; "a discernible change
           in attitude"; "a clearly evident erasure in the
           manuscript"; "an observable change in behavior" [syn:
           discernible, evident, observable]

Moby Thesaurus II by Grady Ward, 1.0:

62 Moby Thesaurus words for "observable":
   apparent, appreciable, beholdable, clear, clear as crystal,
   crystal-clear, detectable, discernible, disclosed, distinct,
   evident, explicit, exposed, exposed to view, express, hanging out,
   in evidence, in full view, in plain sight, in view, indisputable,
   indubitable, insight, manifest, memorable, naked, notable,
   noticeable, obvious, open, open to view, open-and-shut,
   outcropping, palpable, patent, perceivable, perceptible,
   perspicuous, plain, plain as day, recognizable, red-letter,
   revealed, rubric, seeable, self-evident, self-explaining,
   self-explanatory, sensible, showing, tangible, to be seen,
   transparent, unclouded, unconcealed, undisguised, unhidden,
   unmistakable, viewable, visible, visual, witnessable