31 U.S.C. § 1551

(a) In this subchapter— (1) An obligated balance of an appropriation account as of the end of a fiscal year is the amount of unliquidated obligations applicable to the appropriation less amounts collectible as repayments to the appropriation. (2) An unobligated balance is the difference between the obligated balance and the total unexpended balance. (3) A fixed appropriation account is an appropriation account available for obligation for a definite period. (b) The limitations on the availability for expenditure prescribed in this subchapter apply to all appropriations unless specifically otherwise authorized by a law that specifically— (1) identifies the appropriate account for which the availability for expenditure is to be extended; (2) provides that such account shall be available for recording, adjusting, and liquidating obligations properly chargeable to that account; and (3) extends the availability for expenditure of the obligated balances. (c) This subchapter does not apply to— (1) appropriations for the District of Columbia government; or (2) appropriations to be disbursed by the Secretary of the Senate or the Chief Administrative Officer of the House of Representatives.