National debt refers to the total amount of money that a government owes to its creditors, including individuals, corporations, and other countries. It represents the accumulation of a government's borrowing over time, typically through the issuance of government bonds or other forms of debt.

National debt is an important measure of a country's financial health and economic stability, as excessive debt can lead to higher interest rates, inflation, and reduced investor confidence. Additionally, high levels of debt can limit a government's ability to invest in important programs and services, as more and more of its budget goes toward paying off debt and interest payments.

National debt is often measured as a percentage of a country's gross domestic product (GDP), which provides an indication of the country's ability to pay off its debt based on its overall economic output. Countries with higher levels of debt relative to their GDP may face greater economic challenges and are often subject to increased scrutiny from investors and credit rating agencies.