What is a ‘Poison Pill’ defence?

Developed in the 1980s as a defence mechanism strategy that keeps a company from being acquired. It also gives companies more time to evaluate an offer, and can give the board leverage in trying to force a direct negotiation with the potential acquirer

from Moneycontrol Business News https://www.moneycontrol.com/news/business/what-is‘poison-pill’-defence_16279661.html