What is reexamination?

Reexamination is a process where the Patent and Trademark Office, upon the request of a third party, will review a patent in light of some new prior art discovered by a third party and which the PTO had not considered during its initial issuance of the patent. For example, if you were sued by someone for patent infringement, one of the things you would likely do is conduct a search of the relevant prior art for any prior patents that could help you defend your case. If you find something which you believe invalidates the patent and which was not considered by the PTO, you can request that the PTO reexamine the patent in light of this new prior art.