A promissory note is a legal document that is a written promise for one party to pay another party a certain amount of money by a certain timeframe. They are essentially loan agreements.
Promissory notes are often used as loans and mortgages; they can also be used to make payments for services rendered and other obligations. The main terms included are the loan amount, interest rate, payment deadline, and whether it will be secured by collateral.
An addendum to a promissory note changes the terms of the original promissory note.