veto

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The constitutional power of the chief executive of a state or nation to prevent or delay the enactment of legislation passed by the legislature.
• noun An instance in which this right is exercised.
• noun An official document or message from a chief executive stating the reasons for rejection of a bill.
• noun The power of one party or entity to forbid the actions or decisions of another party or entity.
• noun A prohibition or rejection of a proposed or intended act.
• transitive verb To prevent or delay (a legislative bill) from becoming law by exercising the power of veto.
• transitive verb To forbid, prohibit, or decide against.

from The Century Dictionary.

• To forbid authoritatively; specifically, to negative by exercising the constitutional right of veto: as, to veto a bill.
• noun In a constitutional government, the right vested in one branch of it to negative the determinations of another branch; specifically, the right, under constitutional restrictions, of the executive, as a king, a president, or a governor, to reject a bill passed, by the legislature; also, the act of exercising this right.
• noun Any right or power of authoritatively forbidding or effectively negativing, or the exercise of such right or power; prohibition; interdict.

from the GNU version of the Collaborative International Dictionary of English.

• transitive verb To prohibit; to negative; also, to refuse assent to, as a legislative bill, and thus prevent its enactment.
• noun An authoritative prohibition or negative; a forbidding; an interdiction.
• noun 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.
• noun The exercise of such authority; an act of prohibition or prevention.
• noun United States A document or message communicating the reasons of the executive for not officially approving a proposed law; -- called also veto message.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A political right to disapprove of (and thereby stop) the process of a decision, a law etc.
• noun An invocation of that right.
• verb transitive To use a veto against.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun 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)
• noun a vote that blocks a decision
• verb vote against; refuse to endorse; refuse to assent
• verb command against