A memorandum, also known as a memo, is a written message that is typically used in a professional setting. It is a brief and concise communication that is designed to be easily understood. Memos are used for internal communication in a business, while letters are typically used for external communication. A memorandum can serve many important purposes, such as bringing notice to problems and helping to solve a problem through clear and concise communication. Memos also support decision making and can help to ensure that everyone is on the same page.

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 memorandum can be used to communicate a directive, advisory, or informative matter. A specific type of memorandum is the policy briefing note, which is a document for transmitting policy analysis into the political decision-making sphere.

The origins of the term "briefing" lie in legal "briefs" and the derivative "military briefings". The plural form of the Latin noun memorandum is properly memoranda, but if the word is deemed to have become a word of the English language, the plural memorandums, abbreviated to memos, may be used.

In summary, a memorandum is a written message used in a professional setting that is brief and concise, and is typically used for internal communication in a business. It can serve many important purposes, such as bringing notice to problems and supporting decision making.