According to Wikipedia, a Senior Note can be defined as follows: "In finance, senior debt, frequently issued in the form of a senior note, is debt that takes priority over other unsecured debt owed by the issuer. Senior debt has greater seniority in the issuer's capital structure than subordinated debt. In the event the issuer goes bankrupt, senior debt theoretically must be repaid before other creditors receive any payment.
Senior debt is often secured by collateral on which the lender has put in place a first lien. Usually this covers all the assets of a corporation and is often used for revolving credit lines. It is the debt that has priority for repayment in a liquidation."