Simply means to "stop," "block" or "not allow" a party to take a new position that contradicts previous actions or conditions upon which an expectation was created. Such expectation is reasonably relied on by an individual or entity to their detriment. The classic insurance application of estoppel is an insurance carrier that has routinely allowed late payments to be called in from the agent's office; clients accustomed to this allowance depend on its continued practice. The carrier changes position, deciding to no longer accept agent-called-in late payments, but does not notify customers or allow sufficient time for the change to effectuate. A traditionally late-paying customer presents payment at their agent's office only to discover that the insurance carrier will not credit payment nor reinstate (if necessary) until the money is received by the carrier. Coverage cancels and the insured experiences a loss while payment is enroute; the carrier would be estopped from cancelling the policy and denying coverage due to the client's reasonable reliance upon the carrier's previous position/actions. Two broad classes of estoppel within which other types of estoppel exist include: 1) equitable estoppel; and 2) collateral estoppel.
• Equitable Estoppel: prevents a person from adopting a new position that contradicts a previous position created by words, silence, or actions if allowing the adoption of the new position would unfairly harm the person who relied on the previous position to his or her detriment. Also known as estoppel in pais; and
• Collateral Estoppel: a judgment of fact found in one case prevents a party from trying to deny that fact in another case. The parties are permanently bound by the prior ruling. If Elliot Spitzer were tried and found guilty of malicious prosecution in his criminal trial (a question of "fact"), he would be estopped (collaterally) from denying guilt in any trial that follows; his guilt would be considered a foregone conclusion decided by a previous jury and undeniable in any succeeding case (example only, not intended to imply or suggest any actual suit).