Promissory estoppel is a legal principle that prevents a party from withdrawing a promise when the other party has relied on that promise to their detriment. It typically arises in situations where a promise is made without a formal contract, but the promisee takes action based on that promise, leading to potential harm if the promise is not fulfilled. To establish promissory estoppel, three key elements must be present: a clear and definite promise, reliance on that promise by the other party, and resulting harm or injustice if the promise is not honored. This principle helps ensure fairness in situations where formal agreements are lacking.