Definition of 'Estoppel'
A legal defense tool used when someone reneges on or contradicts a previous agreement or claim. Estoppel prevents someone from arguing something contrary to a claim made or act performed by that person previously. Conceptually, estoppel is meant to prevent people from being unjustly wronged by the inconsistencies of another person's words or actions.
Investopedia explains 'Estoppel'
There are many different types of estoppel. Two common forms are equitable estoppel, which can prevent a person from going back on his word, and collateral estoppel, which can prevent a person from going back to court on the same grievance. Collateral estoppel is used to prevent legal harassment and abuse of legal resources.
For example, if a mother states that a child is not hers, estoppel could prevent her from later trying to claim child support payments from the child's father.