Postscriptum

A postscript (P.S., PS, PS.) 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. The term comes from the Latin post scriptum, an expression meaning "written after" (which may be interpreted in the sense of "that which comes after the writing").
In a book or essay, a more carefully composed addition (e.g., for a second edition) is called... meer