Estoppel is a judicial tool allowing a court to prevent, or "estop" a person from making assertions or from going back on the person's word.  The person being sanctioned is "estopped."

So, for instance, when person A, by act or words, gives person B reason to believe a certain set of facts, and person B takes action based on that set of facts, person A cannot later, to person A's benefit, deny those facts or say that person A's earlier act was improper.

Estoppel has been defined by the Utah Supreme Court as: