Debt ceiling
A statutory cap on total federal borrowing. It limits paying for spending already authorised, not new spending.
The debt ceiling is a limit on the stock of debt Treasury may have outstanding. It is set in statute and must be raised or suspended by an act of Congress.
The common misreading is that raising it authorises new spending. It does not. Spending and revenue are decided in separate legislation; the ceiling governs whether Treasury may borrow to settle obligations Congress has already created. Reaching it does not cancel those obligations — it removes the means of paying for them.
When the cap binds, Treasury uses accounting steps known as extraordinary measures to keep total debt under the limit, which buys weeks or months before the point at which cash runs short.
It appears whenever total debt approaches the cap and Treasury begins using 'extraordinary measures' to stay under it.