The American Heritage® Dictionary of the English Language, 4th Edition
adj. Of, relating to, or constituting measures or statutes intended to prevent acquisition of a target company by another company hostile to the target's management.
"One major wild card in a possible deal is activist investor Ronald Burkle , who has a 19.74% stake, just below the 20% threshold at which Barnes & Noble's "poison pill" antitakeover provision kicks in."