A memorandum (pl.: memorandums or memoranda; from the Latin memorandum, "(that) which is to be remembered"), also known as 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.

A specific type of memorandum is the policy briefing note (alternatively referred to in various jurisdictions and governing traditions as policy issues paper, policy memorandums, or cabinet submission amongst other terms), a document for transmitting policy analysis into the political decision making sphere. Typically, a briefing note may be denoted as either “for information” or “for decision”.

The origins of the term lie in legal “briefs” and the derivative “military briefings”. The plural form of the Latin noun memorandum so derived 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. (See also Agenda, Corrigenda, Addenda).

There are many important purposes of a memorandum. Bringing notice to problems, and helping to solve a problem through clear and concise communication are two. Memos support decision making and to “help (or sometimes influence) a decision-maker to make a better decision in a particular problem situation than he might otherwise have made without the analysis”. Other purposes that the briefing note can serve include: conveying information; informing decisions, making a request, providing a response to a question, making a suggestion, presenting an informal report, proposing a solution to a problem, or documenting a reference for future use. Memorandums can be used to make brief appeals or give suggestions. These actions in a brief paper can help significantly expedite business actions to make a positive impact in an organization.

As the communication mechanism of the policy analysis process, the briefing note should provide a coherent synopsis of a policy problem, identify different policy options for addressing the problem, articulate opposing perspectives and advocate a recommended option. The typical structure for a briefing note includes a description of the proposed policy; relevant background information; a discussion of key considerations (including implementation concerns, financial considerations, stakeholder impacts, and possible unanticipated consequences), a summary of arguments for and against the policy and a recommended decision. Policy documents that start with a proposal and assemble an argument for that position are more accurately referred to as a government white paper. A government green paper which raises a policy option and is meant to open a dialogue on the proposal is more similar in tone to a briefing note than is a white paper.

A memo's concise format is relatively standardized in order to create accessibility to any reader. They open with a heading including a "to," "from," "date," and "subject". A break in the text would then be followed by an opening paragraph, which would more than likely describe the purpose of the memo. Context is then added to the document, followed by a section of specific actions. Examples could include "You asked that I look at...." or "To determine the best method of promoting the new fall line, I will....". Actions are followed by discussion, which is typically the longest part of a memo, before concluding the message.