Poison pill

The most famous anti-take-over device. It normally takes the form of granting existing stockholders (other than stockholders who acquire more than a certain percentage of the company) the option (which can only be exercised upon certain events) to buy more stock on very favourable terms as a way of diluting the position of the person trying to take control.

See anti-dilution provisions, anti-dilution (full ratchet), anti-dilution (weighted average), blank cheque preferred stock, shark repellent.