In voting methods, 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 their sincere preference in order to prevent an undesirable outcome - wikipedia 
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.
It has been shown by the Gibbard–Satterthwaite theorem that any single-winner 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 method to another.