What is Swing Vote?

Legal Definition
A swing vote is a vote that is seen as potentially going to any of a number of candidates in an election, or, in a two-party system, may go to either of the two dominant political parties. Such votes are usually sought after in elections, since they can play a big role in determining the outcome.

A swing voter or floating voter is a voter who may not be affiliated with a particular political party (Independent) or who will vote across party lines. In American politics, many centrists, liberal Republicans, and conservative Democrats are considered "swing voters" since their voting patterns cannot be predicted with certainty.

While the swing voter is ostensibly the target of most political activity during elections, in countries without compulsory voting the political parties know that the shift from one party to another is dependent only to a limited extent on swing voters. Another, arguably larger factor is the success of one party in comparison to another in getting out its core support. In a two-party system, those who become disillusioned with their favored party are more likely to vote third-party or abstain than cross over.

Smaller groups with voting powers, such as chambers of parliament and supreme courts, can also have swing voters. Due to these groups' smaller size, an individual swing voter can hold more power. (For example, on a court of seven judges, of which three are committed to each side of a case, the seventh judge may be seen as single-handedly deciding the case.)
-- Wikipedia