Real Dictionary

postscript
countable noun: A postscript is an addition to a finished story, account, or statement, which gives further information.
countable noun: A postscript is something written at the end of a letter after you have signed your name. You usually write ' PS' in front of it.

postscript in American English
a note, paragraph, etc. added below the signature in a letter or at the end of a book, speech, etc. as an afterthought or to give supplementary information
noun: a page description language using scalable fonts that can be printed on a variety of appropriately equipped devices, including laser printers and professional-quality imagesetters
noun: a paragraph, phrase, etc., added to a letter that has already been concluded and signed by the writer

postscript in British English
noun: a message added at the end of a letter, after the signature