The Collaborative International Dictionary of English v.0.48:

overt \o"vert\ ([=o]*v[~e]rt" or [=o]"v[~e]rt), a. [OF. overt,
   F. ouvert, p. p. of OF. ovrir, F. ouvrir, to open, of
   uncertain origin; cf. It. aprire, OIt. also oprire, L.
   aperire to open, operire to cover, deoperire to uncover.
   Perh. from L. aperire influenced by F. couvrir to cover. Cf.
   Aperient, Cover.]
   [1913 Webster]
   1. Open to view; public; apparent; manifest. Opposite of
      hidden.
      [1913 Webster]

            Overt and apparent virtues bring forth praise.
                                                  --Bacon.
      [1913 Webster]

   2. (Law) Not covert; open; public; manifest; as, an overt act
      of treason. --Macaulay.
      [1913 Webster]

            No person shall be convicted of treason unless on
            the testimony of two witnesses to the same overt
            act, or on confession in open court. --Constitution
      of the U. S.
      [1913 Webster]

   Note: In criminal law, an overt act is an open act done in
         pursuance and manifestation of a criminal design; the
         mere design or intent not being punishable without such
         act. In English law, market overt is an open market; a
         pound overt is an open, uncovered pound.
         [1913 Webster]

WordNet (r) 3.0 (2006):

overt
    adj 1: open and observable; not secret or hidden; "an overt
           lie"; "overt hostility"; "overt intelligence gathering";
           "open ballots" [syn: overt, open] [ant: covert]

Moby Thesaurus II by Grady Ward, 1.0:

22 Moby Thesaurus words for "overt":
   apparent, bald, bare, clear, clear-cut, disclosed, evident,
   exposed, manifest, naked, observable, obvious, open, open as day,
   open to all, patent, plain, public, revealed, unclassified,
   unconcealed, visible



Bouvier's Law Dictionary, Revised 6th Ed (1856):

OVERT. Open. An overt act in treason is proof of the intention of the
traitor, because it opens his designs; without an overt act treason cannot
be committed. 2 Chit: Cr. Law, 40. An overt act then, is one which manifests
the intention of the traitor, to commit treason. Archb. Cr. Pl. 379 4 Bl.
Com. 79.
     2. The mere contemplation or intention to commit a crime; although a
sin in the sight of heaven, is not an act amenable to human laws. The were
speculative wantonness of a licentious imagination, however dangerous, or
even sanguinary in its object, can in no case amount to a crime. But the
moment that any overt act is manifest, the offender becomes amenable to the
laws. Vide Attempt; Conspiracy, and Cro. Car. 577.