National Debt
Plain definition
The total accumulated amount a government owes from all past budget deficits minus surpluses; the stock of outstanding government borrowing.
In economics
The national debt is a stock, not a yearly figure: each year's deficit adds to it. What matters economically is the debt-to-GDP ratio and the interest burden, not the raw dollar total. Governments that borrow in their own currency rarely 'run out of money,' but high debt can raise interest rates and crowd out private investment.
Example
If a country has run deficits totaling 4 trillion in surpluses along the way, its national debt is roughly $26 trillion. Annual interest payments on that debt then become one of the largest budget items, competing with defense and social programs.
The national debt is confused with the budget deficit. The deficit is this year's overspending (flow); the debt is the running total owed (stock). Cutting the deficit in half does not cut the debt in half, it just makes the debt grow more slowly.