Which term describes a short informal letter or written message?

The term that describes a short informal letter or written message is "note." Notes are typically concise and can be used for a variety of purposes, such as conveying quick information, reminders, or personal messages. Their informal nature distinguishes them from more structured forms of communication, making them appropriate for casual interactions or brief exchanges of ideas.

An index refers to a systematic list of items, usually found at the end of a book, that helps readers locate specific information. A list is a collection of items that are typically organized in a sequence, and while it might serve a purpose similar to a note in terms of brevity, it lacks the personal touch or message conveyed in a note. An annotation involves adding notes or comments to a text for clarification or explanatory purposes, but it is not used as a standalone message like a note is. Therefore, "note" is the most accurate term for a short informal letter or written message.