What is a "terminal disclaimer"?

A terminal disclaimer is a voluntarily-made binding statement that causes a second patent to expire on the same date as a first patent. Where two patent applications by the same inventor or joint inventors have been found to include substantially the same invention, and a finding of double patenting has been made, the filing of a terminal disclaimer allows the second patent to issue with an expiration date that coincides with the expiration date of the first patent. The voluntary curtailment of the term of the second patent removes any concern that the existence of two similar patents will improperly extend the duration of the patent monopoly beyond the twenty year term.

Terminal disclaimers can be used to get around obviousness-type double patenting rejections, but they cannot be used to get around same-invention double patenting rejections.