Estoppel is a legal principle that prevents a person from arguing something contrary to a claim they previously made or accepted as true. It is often used to uphold fairness and consistency in legal proceedings. For example, if someone has made a promise and another person has relied on that promise, the first person may be "estopped" from denying the promise later. There are different types of estoppel, including promissory estoppel and equitable estoppel. Promissory estoppel applies when a promise leads someone to take action, while equitable estoppel prevents a party from taking a position that contradicts their previous statements or actions. Both types aim to protect individuals from unfair treatment.