Source : Webster's Revised Unabridged Dictionary (1913)

Veto \Ve"to\, n.; pl. {Vetoes}. [L. veto I forbid.]
   1. An authoritative prohibition or negative; a forbidding; an
      interdiction.

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

   2. Specifically:
      (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}.
      (b) The exercise of such authority; an act of prohibition
          or prevention; as, a veto is probable if the bill
          passes.
      (c) A document or message communicating the reasons of the
          executive for not officially approving a proposed law;
          -- called also {veto message}. [U. S.]

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

Veto \Ve"to\, v. t. [imp. & p. p. {Vetoed}; 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.

Source : WordNet®

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: {blackball}, {negative}]
     2: command against; "I forbid you to call me late at night";
        "Mother vetoed the trip to the chocolate store" [syn: {forbid},
         {prohibit}, {interdict}, {proscribe}, {disallow}] [ant: {permit},
         {permit}]
     [also: {vetoes} (pl)]