U might have used this

A X may be a sentence, a 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 X, is written in response to critical remarks on the first edition. The word "X" has, poetically, been used to refer to any sort of addendum to some main work, even if not attached to a main work, as in Soeren Kierkegaard's book titled Concluding Unscientific X

The answer is P.S.(Postscript) a great crack by Abhinav