A memorandum entry is a short message entered into the general journal and also entered into a general ledger account. It is not a complete journal entry because it does not contain debit and credit amounts.
An example of a memorandum entry might be the following:
"On May 1, 2013 a 2-for-1 stock split was declared for the common stockholders of record as of the end of the day May 22, 2013. The stock split will result in the number of issued and outstanding shares of common shares increasing from 200,000 shares to 400,000 shares."
Since a stock split does not change the balance in the Common Stock account, a complete journal entry was not required. The memorandum entry merely notes for future reference that the number of shares of stock has changed.