(1) New facts or arguments raised in a pleading that go beyond just denying the allegations and present new issues. Federal Rule of Civil Procedure, Rule 15 governs when parties may amend, thereby incorporating the new matter into their pleadings. As a general matter under Rule 15, as the proceedings approach trial, it is more difficult for a party to amend their pleadings.
(2) In the context of patent prosecution, new matter is additional information added to an patent. 35 U.S.C. §§ 251-256 lays out the process for patent holders to amend their patents, potentially adding new matter. However, after an applicant’s patent is rejected though and the applicant seeks reexamination of his application, 35 U.S.C. § 132 prohibits amendments to the patent which introduce new matter.
[Last updated in December of 2020 by the Wex Definitions Team]