A memorandum (often shortened to memo) is a written message used inside an organization to share information, give instructions, or make announcements. If your school principal wants to tell all the teachers about a schedule change, she might write a memorandum. If a company manager needs to update the team about a new policy, he'll send a memo.

Memorandums are more formal than quick notes or text messages, but less formal than official letters. They typically include a clear subject line, the date, and who it's from and who it's to. A memo gets straight to the point: “Here's what you need to know” or “Here's what we're doing.”

You might see memorandum of understanding when two groups write down what they've agreed to do together. The plural can be either memorandums or memoranda, though memorandums is more common in everyday use.