A more thorough explanation:

A terminal disclaimer is a statement made by a patent applicant that shortens the term of the patent. This is done to avoid a double-patenting rejection, where the same invention is claimed in two separate patents. The inventor agrees that both patents will expire on the same day to avoid this issue.

For example, if an inventor has a patent for a new type of phone and then applies for a patent for a new feature on that phone, they may need to file a terminal disclaimer to ensure that both patents expire at the same time.

Another example is when an applicant wants to revive an abandoned patent application. They must disclaim a period equal to the duration of abandonment to do so.

Overall, a terminal disclaimer is a legal statement that helps to ensure that patents are not granted for the same invention twice and that the patent term is consistent across related patents.