Debt is the water already in the borrowing tank; a deficit is an additional flow into it over a period.
Go deeper
For the U.S. federal government, debt is an accumulated amount measured at a point in time. A deficit occurs during a fiscal period when spending exceeds revenue. Borrowing helps finance that gap. The two measures are related, but a smaller annual deficit can still accompany a rising debt total.
Federal debt includes debt held by the public and intragovernmental holdings, such as Treasury securities held by federal trust funds. These categories should not be silently interchanged. Interest payments are a cost associated with borrowing, separate from the outstanding principal. A useful comparison identifies which government, which debt definition and which date it covers.
Make it concrete
Hypothetical example: annual spending is $6 trillion and revenue is $5 trillion, producing a $1 trillion deficit. If the next deficit falls to $800 billion, the government still has a financing gap.
When you read a headline
Distinguish debt from deficit and total federal debt from debt held by the public. A smaller deficit does not mean outstanding debt has fallen by that amount.