Postscript \Post"script\, n. [L. postscriptus, (assumed) p. p. of postscribere to write after; post after + scribere to write: cf. F. postscriptum. See Post-, and Scribe.]

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. [Abbrev. P. S.]

Source: Webster's Revised Unabridged Dictionary (1913)