A brief written note or record kept for future reference, often as a personal reminder or informal documentation.
I summarized the meeting content in a memo.
I jotted down the phone number as a note so I wouldn't forget.
Compound of 控え (ひかえ, 'reserve, copy, note') + 書き (かき, 'writing'). The exact historical derivation is uncertain, but the word literally suggests a written copy or note kept in reserve.