In patent law, a disclaimer are words identifying, in a claim, subject-matter that is not claimed. By extension, a disclaimer may also mean the amendment consisting in introducing a negative limitation in an existing claim, i.e. "an amendment to a claim resulting in the incorporation therein of a 'negative' technical feature, typically excluding from a general feature specific embodiments or areas". The allowability of disclaimers is subject to particular conditions, which may vary widely from one jurisdiction to another.
Origins
Possibly the earliest mention of patent disclaimers was in the British "Letters Patent and Trademark Amendment Act 1835", in the sense of a right to renounce one's patent monopoly or a part thereof. That right was subject to safeguards to make sure that the disclaimer was a true renunciation, rather than an extension of the monopoly. In 1865, in a case before the House of Lords, Ralston v. Smith (XI H.L.C. 223), the applicability of these safeguards were illustrated. Namely, an amendment by the patentee extended beyond the content of the patent application as filed, therefore extended the patent monopoly, and was not allowed by the House of Lords.