A strategic move by a takeover target to make its stock less attractive is called a poison pill.
More precisely, a poison pill is a takeover-defense plan—often called a shareholder rights plan—designed to make acquiring control of a company much more expensive or difficult. A common version lets shareholders other than the bidder buy additional shares at a steep discount if the bidder crosses a preset ownership threshold. This dilutes the bidder’s stake and raises the cost of the takeover.
The term became prominent in the early 1980s, when hostile takeovers were widespread in the United States. Poison pills generally do not make the company’s stock permanently unattractive to ordinary investors; they make an unsolicited acquisition unattractive to a particular bidder. Their usual purpose is to pressure the bidder to negotiate with the target’s board or to give management time to seek a better offer.
A poison pill is one of several anti-takeover defenses. Others include a staggered board, a white-knight merger, a crown-jewel defense, and a Pac-Man defense. The phrase can also describe broader tactics, but “poison pill” is the standard canonical answer to this question.