A casting vote is a vote that someone may exercise to resolve a deadlock. A casting vote is typically by the presiding officer of a council, legislative body, committee, etc., and may only be exercised to break a deadlock.
Examples of officers who hold casting votes are the Speaker of the British House of Commons and the President of the United States Senate (an ex-officio role of the Vice President of the United States).
In some legislatures, a casting vote may be exercised however the presiding officer wishes. An example is the Vice President of the United States, who may exercise his casting vote in the Senate according to his party affiliation or according to his own personal beliefs; by virtue of the Vice President's casting vote, when the Senate as elected is equally divided between two parties, the Vice President's party is able to serve as the official majority party in the Senate.
In some other legislatures, by contrast, a casting vote can only be exercised according to strict rules or conventions. For example, the Speaker of the British House of Commons (and analogous positions in most Westminster Parliaments) is expected by constitutional convention to follow Speaker Denison's rule, i.e. to vote to allow further discussion, if this is possible, and otherwise to vote in favour of the status quo.
In the United States, the concept of a casting vote is not used in Robert's Rules of Order.
Under some rules of parliamentary procedure, notably Robert's Rules of Order, the presiding officer does not have a casting vote in the way it is normally understood as a means to break ties. Instead, the officer has a normal vote but exercises it only after other members have voted and only if it would make a difference. That allows the presiding officer to vote against a motion to bring it to a tie and defeat it (for instance, if the vote is 50-49, the presiding officer could defeat the vote by voting against) in addition to breaking a tie by voting in favour. The intent behind the rule is to give the presiding officer the same voting rights as other but while preserving their impartiality whenever possible by not having them vote unless it would change the outcome.
Some legislatures have abandoned the concept of a casting vote. For example, the Speaker of the New Zealand House of Representatives formerly held a casting vote similar to that of the Speaker of the British House of Commons. Today, however, the Speaker simply votes as an ordinary member, and since an outright majority is necessary for a bill to pass, a tie is considered a defeat.
Some legislatures have a dual approach. In the Australian Parliament, the Speaker of the House of Representatives may not vote in general debates but has a casting vote to decide a tie. The President of the Senate usually votes in general debates, which are commonly based on party lines. The President does not have a casting vote, and a tied vote in the Senate is resolved in the negative. The same arrangements exist with respect to the Speakers of the Canadian House and Senate.
While having the same right to vote as any member of the House, the Speaker of the United States House of Representatives, to maintain the appearance of impartiality, typically does not vote unless doing so would make a difference, which is, in effect, a casting vote.
In the Congress of the Philippines, the openly-partisan presiding officers of the two chambers have different rules on casting votes in case of a tie. In the Senate, the President of the Senate votes last; therefore, if the motion is tied, it is lost. Meanwhile, the Speaker of the House of Representatives (or any presiding officer) does not vote unless there is a tie, which is rare; in such a casting vote, the presiding officer usually votes based on party line.
At one time, in United Kingdom parliamentary elections, the Returning Officer (if an elector in the constituency) was allowed to give an additional casting vote to decide the election if there was a tie between two or more candidates. An example of this power being used was in the Bandon by-election of 22 July 1831. This type of casting vote does not now exist: ties in United Kingdom elections are now broken by drawing lots, using a method decided upon by the Returning Officer.
In the Canadian provinces of Ontario and New Brunswick, the returning officer still has a casting vote in the event of a tie. Ties in Nova Scotia and Prince Edward Island, as well as the territory of Yukon, are now broken by lots as in the United Kingdom. In the remaining provinces and territories, as well as in federal elections, a tie vote results in a by-election held to elect a new member (who need not have been a candidate in the first election).