postface

n. postscript, additional text added to a written work

A postface is the opposite of a preface, a brief article or explanatory information placed at the end of a book. Sometimes general information about a book and the people for whom it was written is at the back of the book in a postface. In ancient Chinese works, the postface is called 序/叙言. Afterwords are quite often used in books so that the non-pertinent information will appear at the end of the literary work, and not confuse the reader.
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n. postscript, P.S., additional text added to a letter that has been completed and signed; additional note, additional text supplying additional information