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:

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Johannessen v. Canyon Road Towers (modification of assessment, ownership interests, estoppel) '02
Lodge at Westgate Park City v. Westgate Resorts (standing to represent owners, promissory estoppel, ratification, common area definition)
Rowley v. Marrcrest HOA (waiver, estoppel, ARC review, self help) '82
HOA resources and laws annotated
HOA resources and laws annotated