A memorandum (PL: memoranda; abbr: memo; from the Latin memorandum, "(that) which is to be remembered") or a briefing note is a written message that is typically used in a professional setting. Commonly abbreviated "memo," these messages are usually brief and are designed to be easily and quickly understood. Memos can thus communicate important information efficiently in order to make dynamic and effective changes.

In law, a memorandum is a record of the terms of a transaction or contract, such as a policy memo, memorandum of understanding, memorandum of agreement, or memorandum of association. In business, a memo is typically used by firms for internal communication, while letters are typically for external communication.

Other memorandum formats include briefing notes, reports, letters, and binders. They may be considered grey literature. Memorandum formatting may vary by office or institution. For example, if the intended recipient is a cabinet minister or a senior executive, the format might be rigidly defined and limited to one or two pages. If the recipient is a colleague, the formatting requirements are usually more flexible.