A paragraph added to a letter after it is concluded and signed by the writer; an addition made to a book or composition after the main body of the work has been finished, containing something omitted, or something new occurring to the writer.
A postscript (from post scriptum, a Latin expression meaning "after writing" and abbreviated P.S.) is a sentence, paragraph, or occasionally many paragraphs added, often hastily and incidentally, after the signature of a letter or (sometimes) the main body of an essay or book. In a book or essay, a more carefully-composed addition (e.g., for a second edition) is called an afterword. An afterword, not usually called a postscript, is written in response to critical remarks on the first edition.