noun
A personal note or informal record written to remember something.
会議の内容を覚書にまとめた。
I summarized the meeting content in a memo.
忘れないように覚
I stuck a note on my desk so I wouldn't forget.
noun
diplomatic memorandum
Only when written 覚書
A formal diplomatic document; usually written as 覚書.
両国は貿易に関する覚書を交わした。
The two countries exchanged a memorandum on trade.