you likely already know it, it is usually abbreviated "P.S."
► uses
Uses:
'Exterminate all the brutes!' The curious part was that he had apparently forgotten all about that valuable postscriptum, because, later on, when he in a sense came to himself, he repeatedly entreated me to take good care of 'my pamphlet' (he called it), as it was sure to have in the future a good influence upon his career.