poison pill 
noun Finance
A tactic used by a company threatened with an unwelcome takeover bid to make itself unattractive to the bidder: [as modifier]: poison-pill defences...
- The next time you read a news release that says that your company is using a poison pill to ward off a takeover attempt, you'll now know what it means.
- Oracle has said it will not go forth with the takeover with the poison pill in place.
- At heavy-duty truck producer Navistar International in Warrenvilie, Ill., for example, a resolution to rescind the poison pill was supported by 83 percent of the quorum.