From Wikipedia: Estoppel.
Estoppel in its broadest sense is a legal term referring to a series of legal and equitable doctrines that preclude “a person from denying or asserting anything to the contrary of that which has, in contemplation of law, been established as the truth, either by the acts of judicial or legislative officers, or by his own deed, acts, or representations, either express or implied.”
Because estoppel is so factually dependent, it is perhaps best understood by considering specific examples such as the following:
Example 1: A city entered into a contract with another party. The contract stated that it had been reviewed by the city’s counsel and that the contract was proper. Estoppel applied to estop the city from claiming the contract was invalid.
Example 2: A creditor unofficially informs a debtor that the creditor forgives the debt between them. Even if such forgiveness is not formally documented, the creditor may be estopped from changing its mind and seeking to collect the debt, because that change would be unfair.
Example 3: A landlord informs a tenant that rent has been reduced, for example, because there was construction or a lapse in utility services. If the tenant relies on this statement in choosing to remain in the premises, the landlord could be estopped from collecting the full rent.