U.S. patent law Edit
An opposition can also be the actual proceedings where a third party raises their objection to the grant of a particular patent.
U.S. trademark law Edit
An opposition is a proceeding before the Trademark Trial and Appeal Board in which the plaintiff seeks to prevent the issuance of a registration of a mark. An opposition is similar to a proceeding in a federal court, but is held before the Trademark Trial and Appeal Board, a U.S. Patent and Trademark Office administrative tribunal.
Any person who believes that he or she will be damaged by the registration of a mark may file an opposition, but the opposition may only be filed in response to the publication of the mark in the Official Gazette.