ETYM Latin, something to be remembered, neut. of memorandus, fut. pass. p. of memorare. Related to Memorable.
(Irregular plural: memorandums, or: memoranda).
1. A record of something which it is desired to remember; a note to help the memory.
2. A brief or informal note in writing of some transaction, or an outline of an intended instrument; an instrument drawn up in a brief and compendious form.
Or memo written note or message giving information or issuing instructions. It is usually short and details who is sending the memo and to whom it should be distributed.