From Academic Kids
A split vote, or vote splitting, occurs in an election when the existence of two or more candidates that represent relatively similar viewpoints among voters reduces the votes received by each of them, reducing the chances of any one of them winning against another candidate, who represents a significantly different viewpoint. These can lead to a candidate that represents the viewpoints of a minority of voters winning.
Vote splitting as an issue is usually confined to first-past-the-post voting systems such as those used by the United Kingdom, the United States and Canada - it is not generally an issue in countries which use proportional representation such as Germany. Preferential voting systems like the one in Australia also tend to eliminate vote splitting as an issue.
In the United States, a famous example of a split vote occurred in the 2000 U.S. Presidential Election when Green Party candidate Ralph Nader attracted voters who might otherwise have voted for Democratic Party candidate Al Gore because of the similar liberal platforms of both candidates. Because of the very narrow margin of victory of Republican Party candidate George W. Bush over Gore, many blamed Nader's candidacy for causing his loss and thus being a spoiler (although the votes that went to the eighth-place candidate in the contested state of Florida could have potentially covered the split).
Vote-splitting is a special case of strategic nomination.