What is the difference between memoir and diary?
Senior Member2,018
A diary is a daily accounting of events and thoughts, usually informal in nature.

A memoir is a more formal composition, often intended for publication.

A good dictionary would provide you with this information.
Veteran Member24,100
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A diary is a record of (unrelated) events in one's life whereas a memoir selects specific events or scenes and weaves them into a thematic story. You could write a memoir about your dog, for example, and another memoir about your mother-in-law. In either case you select certain stories or themes and weave them into a narrative.
A diary tells what happens within a specific time frame and is written about specific things that have happened. It doesn't address the person to whom these things happened. Diaries are usually written as a record of events, transactions or observations kept daily or at frequent intervals.

Memoirs are not personal narratives. Memoirs are not single moments. Memoirs are about the plot lines or patterns that bind those moments together.
In addition to what my friends said, I think you can have diary about future both memoir is always about past.

Senior Member2,271
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