strategic voting; tactical voting
Voting not for one's preferred candidate but to prevent an undesirable outcome, common in political science and election contexts.
Strategic voting tends to occur in single-member district systems.
死票 refers to wasted votes that do not contribute to electing a candidate, while 戦略的投票 is the act of voting strategically to avoid wasting one's vote.
Compound of 戦略 (strategy) + 的 (-like, -al) + 投票 (voting). A modern term likely calqued from English 'strategic voting'.