Borrowed money that a company must repay first if it goes out of business. If a company goes bankrupt, senior debtholders, who are often bondholders or banks that have issued revolving credit lines, are most likely to be repaid.
Senior debt is secured by collateral, and that collateral can be sold to repay the senior debt holders. As such, senior debt is considered lower risk and carries a relatively low interest rate. Even though senior debtholders are the first in line to be repaid, they will not necessarily receive the full amount they are owed in a worst-case scenario.