Legal estoppel is a principle of law, particularly United States patent law, that an assignor or grantor is not permitted subsequently to deny the validity of title to the subject matter of the assignment or grant.
Originally a principle of real property law, applicable to deeds of land and called estoppel by deed, the Supreme Court extended legal estoppel to patents in Westinghouse Elec.
His explanation for the principle was as follows: [T]here seems to be no reason why the principles of estoppel by deed should not apply to assignment of a patent right….The grantor purports to convey the right to exclude others, in the one instance, from a defined tract of land, and in the other, from a described and limited field of the useful arts.
To determine such an issue, it is admissible to show the state of the art involved, that the court may see what the thing was which was assigned, and thus determine the primary or secondary character of the patent assigned, and the extent to which the doctrine of equivalents may be invoked against an infringer.
The court will not assume against an assignor, and in favor of his assignee, anything more than that the invention presented a sufficient degree of utility and novelty to justify the issuance of the patent assigned….