Tactical voting
Tactical voting happens when a person votes for someone other than their favorite candidate in a way to either help their favorite or achieve some other goal. This may happen in national or local elections, or even in elections for clubs and homeowner associations.
Examples
Say 5 people are running as candidates for board members, but only 3 will be elected. For the following situations except the last, let' s say you get 1 vote.
Your favorite is almost guaranteed to win a seat on the board, so you may choose to vote for your second favorite candidate to help him out. That gives your two favorites a better chance of winning.
Your favorite is almost guaranteed to lose, so you may choose to vote for your second favorite candidate to help him out. That gives your second choice a chance of winning even though your favorite may lose.
Your favorite is uncertain to win, so you may choose to vote for a stronger candidate, rather than see someone else win.
You don' t have a favorite, but hate the current leading candidate, but you may choose to vote for somebody with less votes to even out the results between them.
Say you get to vote for 3 candidates and your favorite is in the middle of the pack. You may choose to vote only for your favorite and not use your other 2 votes so that your favorite faces less competition.
Another common situation happens in primary elections, where members of one party may temporarily' cross over' to the other party to support a candidate who is getting fewer votes, hoping their favorite will face weak competition in the main election. This type of negative voting is popular but often considered unethical.
Ethics
Many experts question tactical voting in that it can undermine a democratic outcome. That is, ethical voters may believe it more important how a candidate wins than who wins. Others feel the end justifies the means. Most researchers tend to frown on tactical voting.
Resources
Svensson, Lars-Gunnar (1999). The Proof of the Gibbard-Satterthwaite Theorem Revisited
The Science of Elections, Brams, Herschbach Science Online (2001). Abstract
Other websites
Tactical Voting Can Be a Weak Strategy -- Article on tactical voting within larger strategic considerations [archived]
VotePair. org VotePair is a banding together of the people who started tactical voting online in the 2000 US elections
Voting methods page Includes extensive discussion of strategic voting in a wide range of real and theoretical voting systems
Voting