备忘录（英语：Memorandum，简写为 Memo），意指任何一种能够帮助记忆，简单说明主题与相关事件的书面数据。它源自于拉丁语：memorandum est，由动词 memoro （原义是「提及、回忆、相关的」），所形成的动名词，意为：「这是应该被记住的」。

在商业办公、法律文档或外交谈判中，一些非正式或半正式的文档通常都会被称为备忘录，可以是简单的字条、文档，或是通信记录。备忘录可以有各种形式，也不一定有固定格式，各个机构内部可以自行决定它的格式。在中国大陆政府及国有企业内部的会议中，备忘录被称为会议纪要，有时可以发挥行政上的契约和指令作用。

A memorandum (abbrev.: memo) was from the Latin verbal phrase memorandum est, the gerundive form of the verb memoro, "to mention, call to mind, recount, relate", which means "It must be remembered (that)...". It is therefore a note, document or other communication that helps the memory by recording events or observations on a topic, such as may be used in a business office. 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)