A legal bar, from medieval law French, meaning "stop." For example, assignors are estopped from challenging their assignee's title, and licensees are estopped from challenging their licensor's title: the assignor or licensee sued by the assignee or licensor for infringement cannot defend by (is estopped from) showing that the right is invalid. Hence, the terms "assignor estoppel" and "licensee estoppel." Someone may be estopped without intending or knowing it. Thus, if A leads B to assume that a certain state of affairs exists, and it would be unfair to let A have a change of heart in the light of what has since happened, B's assumption is treated as true: that is, A is estopped from denying its validity.
A general principle of law preventing a party which has, by act or omission, asserted a statement of fact from resiling from that assertion where to do so would cause prejudice to another party relying upon it; occasionally invoked in territorial disputes to prevent a state from disavowing its prior recognition of another state’s sovereignty.