In the context of takeovers, action taken by the target to make itself unattractive to a bidder or potential bidder. Such action is restricted under the UK City Code on Takeovers and Mergers (www.practicallaw.com/A34660) (Rule 21) and is more common in the US.
For further information, see Practice note, Defensive tactics and the legal and regulatory constraints: Poison pills (www.practicallaw.com/3-107-3621).