Re-examination is a request for the patent office to re-examine a pending patent based on prior art not considered by the patent office during the original prosecution. Re-examination may provide benefits to both the patent holder and the accused infringer.
A patent holder may want their patent re-examined in view of prior art asserted, or likely to be asserted, by an accused infringer. Re-examination offers a patent holder an opportunity to present arguments in an ex-parte proceeding as to why the patent claims distinguish over the new prior art, or to amend the claims in a manner that avoids the new prior art while still providing adequate claim coverage.
An accused infringer may choose to have a patent re-examined by the patent office rather than litigate the issue in court which can be very costly and less predictable due to the non-technical nature of juries and judges. Perhaps the best example is the well publicized NTP v. RIM where RIM opted to litigate and ultimately lose in court, and then successfully invalidated all of the NTP patents in a subsequent re-examination proceeding.