The Collaborative International Dictionary of English v.0.48:

Veto \Ve"to\ (v[=e]"t[-o]), n.; pl. Vetoes (v[=e]"t[=o]z). [L.
   veto I forbid.]
   [1913 Webster]
   1. An authoritative prohibition or negative; a forbidding; an
      interdiction.
      [1913 Webster]

            This contemptuous veto of her husband's on any
            intimacy with her family.             --G. Eliot.
      [1913 Webster]

   2. Specifically:
      [1913 Webster]
      (a) A power or right possessed by one department of
          government to forbid or prohibit the carrying out of
          projects attempted by another department; especially,
          in a constitutional government, a power vested in the
          chief executive to prevent the enactment of measures
          passed by the legislature. Such a power may be
          absolute, as in the case of the Tribunes of the People
          in ancient Rome, or limited, as in the case of the
          President of the United States. Called also the veto
          power.
          [1913 Webster]
      (b) The exercise of such authority; an act of prohibition
          or prevention; as, a veto is probable if the bill
          passes.
          [1913 Webster]
      (c) A document or message communicating the reasons of the
          executive for not officially approving a proposed law;
          -- called also veto message. [U. S.]
          [1913 Webster]

   Note: Veto is not a term employed in the Federal
         Constitution, but seems to be of popular use only.
         --Abbott.
         [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Veto \Ve"to\, v. t. [imp. & p. p. Vetoed (v[=e]"t[=o]d); p.
   pr. & vb. n. Vetoing.]
   To prohibit; to negative; also, to refuse assent to, as a
   legislative bill, and thus prevent its enactment; as, to veto
   an appropriation bill.
   [1913 Webster]

WordNet (r) 3.0 (2006):

veto
    n 1: a vote that blocks a decision
    2: the power or right to prohibit or reject a proposed or
       intended act (especially the power of a chief executive to
       reject a bill passed by the legislature)
    v 1: vote against; refuse to endorse; refuse to assent; "The
         President vetoed the bill" [syn: veto, blackball,
         negative]
    2: command against; "I forbid you to call me late at night";
       "Mother vetoed the trip to the chocolate store"; "Dad nixed
       our plans" [syn: forbid, prohibit, interdict,
       proscribe, veto, disallow, nix] [ant: allow,
       countenance, let, permit]

Moby Thesaurus II by Grady Ward, 1.0:

67 Moby Thesaurus words for "veto":
   absolute veto, ban, block, constitute, decline, decree, defeat,
   denial, deny, disallow, disallowance, embargo, enact, enact laws,
   executive veto, filibuster, forbid, get the floor, have the floor,
   interdict, interdiction, item veto, kill, killing, legislate,
   limited negative, limited veto, lobby through, logroll, negative,
   nix, ordain, outlaw, pass, pigeonhole, pocket, pocket veto,
   preclude, preclusion, prevent, prevention, prohibit, prohibition,
   proscribe, proscription, put in force, put through, quash,
   quashing, railroad through, refuse, reject, rejection, roll logs,
   rule against, rule out, senatorial courtesy, stop, stoppage,
   suspensory veto, table, taboo, take the floor, turn down,
   veto message, veto power, yield the floor



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

VETO, legislation. This is a Latin word signifying, I forbid.
     2. It is usually applied to the power of the president of the United
States to negative a bill which has passed both branches of the legislature.
The act of refusing to sign such a bill, and the message which is sent to
congress assigning the reasons for a refusal to sign it, are each called a
veto.
     3. When a bill is engrossed, and has received the sanction of both
houses, it is transmitted to the president for his approbation. If he
approves of it, he signs it. If he does not, he sends it, with his
objections, to the house in which it originated, and that house enter the
objections on their journals, and proceed to reconsider the bill. Coast. U.
S. art. I, s. 7, cl. 2. Vide Story on the Const. Sec. 878; 1 Kent, Com. 239.
     4. The governors of the several states have generally a negative on the
acts of the legislature. When exercised with due caution, the veto power is
some additional security against inconsiderate and hasty legislation, or
where bills have passed through prejudice or want of due reflection. It was,
however, mainly intended as a weapon in the hands of the chief magistrate to
defend the executive department from encroachment and usurpation, as well as
a just balance of the constitution.
     5. The veto power of the British sovereign has not been exercised for
more than a century. It was exercised once during the, reign of Queen Anne.
Edinburgh Rev. 10th vol. 411, &c.; Parke's Lectures, 126. But anciently the
king frequently replied Le roy s'avisera, which was in effect withholding
his assent. In France the king had the initiative of all laws, but not the
veto. See 1 Toull. art. 39; and see Nos. 42, 52, note 3.