A more thorough explanation:

Definition: Ex parte reexamination is a legal process in which the U.S. Patent and Trademark Office reviews an already-issued patent to determine if it is valid based on prior art. This process can be initiated by anyone, including the patent holder or an anonymous informant, at any time during the life of the patent.

Example: An alleged infringer may seek ex parte reexamination of a patent to narrow its scope and avoid liability. During the process, the challenger can produce prior art and respond to the patentee's statements regarding the new prior art, but they are excluded from further participation in the examination process. This process does not employ discovery mechanisms, and witnesses are not examined.

Explanation: Ex parte reexamination is a way to challenge the validity of a patent without going to court. It allows anyone to bring forward evidence that the patent should not have been granted in the first place. The process is limited to the challenger producing prior art and responding to the patentee's statements, without any further participation. This process is different from inter partes reexamination, which allows both parties to participate in the examination process and request a hearing.