Acquiescence:
Acquiescence in the acts of an agent, or who has assumed character, will be equivalent to an express authority.
Estoppel by acquiescence
The doctrine of estoppel by acquiescence may prevail when Party A makes legal notice to Party B of Party A's assertion of a fact or legal principle or claim, and Party B fails to refute, reply to, or to defend (within "a reasonable period of time") against said claim. In this circumstance, Party B is said to have acquiesced to the claim or position taken by Party A, and by acquiescence, generally is considered to have lost the legal right to make a counterclaim.
Apparent Agency (synonym - Agency By Estoppel)
A legal principle that an agent is deemed to have whatever power or authority a person would reasonably infer, either from the principal's representations concerning an agent's authority or from the agent's holding himself out as having proper authority. The principal or insurer may be obligated as if it had expressly granted the authority to the agent.
Collateral estoppel
The principle that any fact or issue that has been decided in a previous suit should not be litigated by the same parties in another suit. The earlier determination is binding, and a party is estopped, or prevented, from disputing the matter