Involves the deliberate deception of a patent office, especially the United States Patent and Trademark Office, in order to obtain a patent. This can result in at least unenforceability and potentially invalidity, legal, and even criminal penalties and the risk of counterclaims.
Fraud on the patent office is sometimes referred to as inequitable conduct, although strictly speaking inequitable conduct has a lower level of proof and does not require the same degree of bad faith and/or deliberate-falsehood (indeed it can be based on silence as to relevant information) as fraud. Patent applicants have a duty to disclose all information that is material to their application, known as the duty of candor. See Unenforceability, Inequitable Conduct.