casting vote

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

• noun The deciding vote of a presiding officer in an assembly or council, exercised when there is a deadlock due to a tie.

from The Century Dictionary.

• noun The vote of a presiding officer in an assembly or council, thrown to decide a question when the votes cast by the members are equally divided.

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

• noun the deciding vote cast by the presiding officer to resolve a tie