Memorandum is a word of Latin origin meaning "short note," "record of events," "brief message." Memo is an informal but widely used and acceptable abbreviation: "This executive begins each day with a memorandum (or memo) to all salesmen." The standard plural of memorandum is either memorandums or memoranda; the plural of memo is memos: "Please stop sending me so many memorandums (or memoranda or memos)."
Dictionary of problem words and expressions. Harry Shaw. 1975.