Estoppel is a legal doctrine that prevents a person from adopting a position, action, or attitude, asserting a fact or a right, or prevents one from denying a fact inconsistent with an earlier position if it would result in an injury to someone else.
There are several types of estoppel;
Collateral estoppel prevents a party to a lawsuit from raising a fact or issue which was already decided against him in another lawsuit. Collateral estoppel is the legal doctrine that holds that the determination of the facts litigated between the parties to a proceeding are binding and conclusive on those parties in any future litigation. It is also referred to as "issue preclusion". The constitutional ban on double jeopardy includes the right to plead collateral estoppel.
Under collateral estoppel, once a court has decided an issue of fact or law necessary to its judgment, that decision may preclude relitigation of the issue in a suit on a different cause of action involving a party to the first case. The collateral estoppel bar is inapplicable when the claimant did not have a 'full and fair opportunity to litigate' the issue decided by the other court. Thus, a claimant can file a federal suit to challenge the adequacy of state procedures. Ordinarily, collateral estoppel is an affirmative defense that must be raised by the party seeking to use it, or else it is waived.
Equitable estoppel prevents one party from taking a different position at trial than she did at an earlier time if the other party would be harmed by the change. Generally, the elements that need to be proved are:
Promissory estoppel prevents a party from acting in a certain way because the first party promised not to, and the second party relied on that promise and acted upon it. It is also sometimes referred to as detrimental reliance. Promissory estoppel is a term used in contract law that applies where, although there may not otherwise be a enforceable contract, because one party has relied on the promise of the other, it would be unfair not to enforce the agreement. Promissory estoppel is used to enforce charitable gift pledges where the charity relies on them.
It arises from a promise which the promisor should reasonably expect to induce action or forebearance of a definite and substantial character on the part of the promisee and which does induce such action or forebearance in binding if injustice can be avoided only by enforcement of the promise.