What is "tactical voting"

In voting systems, tactical voting (or strategic voting or sophisticated voting or insincere voting) occurs, in elections with more than two candidates, when a voter supports another candidate more strongly than his or her sincere preference in order to prevent an undesirable outcome.

For example, in a simple plurality election, a voter might sometimes gain a "better" outcome by voting for a less preferred but more generally popular candidate, while under instant-runoff voting casting an honest vote may cause the winner to change from a voter's second-favorite to their last-place, most-disliked candidate.

It has been shown by the Gibbard–Satterthwaite theorem that any ranked voting method which is not dictatorial must be susceptible to tactical voting. However, the type of tactical voting and the extent to which it affects campaigns and election results can vary dramatically from one voting system to another.