In an effort to maintain our pledge of brief and straightforward answers, I will cheerfully condense the legal definition of estoppel into a few well-chosen paragraphs. Those pursuing a degree in law may want to rely on another source for the bar exam. Estoppel is a very complex legal term dealing with the role of conscience and truth in a court proceeding.
An estoppel is a defense against a party reneging on a previous statement assumed to be a legal truth. Once a statement of fact is entered into a court case, the person who made that statement must stand by its truthfulness. He or she cannot claim a new position in a future business or private dealing. If the other party makes a decision based on the untruthful second statement and a lawsuit ensues, they can claim an estoppel in court against the plaintiff. In order for the estoppel to be considered valid, however, the defendant needs to demonstrate damages stemming from the untruthful statement.
For example, if a landlord tells a tenant that he or she only owes half of the monthly rent because of repairs, the tenant can reasonably treat that as a statement of fact. He or she may write a check for half of the normal monthly rent and assume he or she is in good standing. If the landlord later decides to sue the tenant for not paying the entire amount of the lease, the tenant may claim an estoppel in court. The landlord's verbal agreement to accept half payment should have been recognized as binding, even if the original lease was not changed. The tenant had a reasonable right to change his rent payments based on a perceived truth.
An estoppel is generally a defensive move, not a first strike. The plaintiff in a case can claim a number of reasons why the defendant should honor a contract or pay damages, for example. The defense must demonstrate that the plaintiff's own statements go against established facts in order to claim an estoppel. There must also be evidence that the defendant based his actions strictly on the reliance of truth and that he suffered damages because of it. Enforcement of an estoppel is generally left up to the discretion of the individual judge hearing the case.