Postscript (noun)

1. A message or remark added to a letter after it has been signed and dated.
2. An addition to a book or article after it has been completed.

Origin:

Mid 16th century: from latin post scriptum "written after", from post "after" + scriptum "something written".

Examples:

1. She added a postscript to her letter, saying she hoped to see him soon.
2. In the postscript of the book, the author thanks the readers for their support.
3. The postscript of the speech was a call to action.
4. The postscript of the article provided additional information.
5. The postscript of the letter was a reminder to RSVP.