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.]
1. An authoritative prohibition or negative; a forbidding; an
This contemptuous veto of her husband's on any
intimacy with her family. --G. Eliot.
(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
(b) The exercise of such authority; an act of prohibition
or prevention; as, a veto is probable if the bill
(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.