What is the definition of the word postscript?

Postscript is better known by its initial form, PS, an addendum or afterthought to a completed written communication. The abbreviation should be followed by a period, as with any later or separate additions that are abbreviated as PPS (post-postscript).

Example :

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Dear Jill :

I am going out of town. I will return on Saturday.

Your devoted brother, Bill

PS. Don't forget to feed my cat.

PPS. Be careful using the gate.

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