Definify.com

Webster 1828 Edition

Noun.

A species of history written by a person who had some share in the transactions related. Persons often write their own memoirs.

A history of transactions in which some person had a principal share, is called his memoirs, though compiled or written by a different hand.

The history of a society, or the journals and proceedings of a society; as memoirs of the Royal Society.

A written account; register of facts.

memoir ‎(plural memoirs)

An autobiography; a book describing the personal experiences of an author.
When I retire, I'm going to write my memoirs.
A biography; a book describing the experiences of a subject from personal knowledge of the subject or from sources with personal knowledge of the subject.
James wrote a memoir of his grandmother shortly after she passed away.
Any form of narrative describing the personal experiences of a writer.

A memoir may differ from a simple biography or autobiography by not focusing on the author as the primary subject matter, but on people and events in the subject's life. Emphasis is placed on personal observations about external events.
The plural memoirs is often used to refer to a single work.

book describing the personal experiences of an author

any form of narrative describing the personal experiences of a writer

“memoir” in Merriam-Webster Online Dictionary, Merriam-Webster Online.
“memoir” (US) / “memoir” (UK) in Oxford Dictionaries, Oxford University Press.
“memoir” in Dictionary.com Unabridged, v1.0.1, Lexico Publishing Group, 2006.
“memoir” in Collins American English Dictionary, Collins