“[Acquiescence] is most generally a kind of ‘permission’ given by silence or passiveness.”
“Silence is acquiescence (aka. silent acquiescence and acquiescence by silence) is a related doctrine that can mean, and have the legal effect, that when confronted with a wrong or an act that can be considered a tortious act, where one’s silence may mean that one accepts or permits such acts without protest or claim thereby loses rights to a claim of any loss or damage.”
“In law, acquiescence occurs when a person knowingly stands by without raising any objection to the infringement of their rights, while someone else unknowingly and without malice aforethought makes a claim on their rights. Consequently, the person whose rights are infringed loses the ability to make a claim against the infringer, or succeed in an injunction suit due to the infringer’s conduct. The term is most generally a kind of ‘permission’ given by silence or passiveness.”
“Acquiescence: Conduct recognizing the existence of a transaction and intended to permit the transaction to be carried into effect; a tacit agreement; consent inferred from silence.”
— Legal Dictionary
Also see laches.