GAO Glossary of Terms and Definition (September 2005)
An act appropriating funds in addition to those already enacted in an annual appropriation act. Supplemental appropriations provide additional budget authority usually in cases where the need for funds is too urgent to be postponed until enactment of the regular appropriation bill. Supplementals may sometimes include items not appropriated in the regular bills for lack of timely authorizations.
Congressional Budget Process: An Explanation (Senate Budget Committee)
Definition of Supplemental Appropriation
Supplemental Appropriation: An act appropriating funds in addition to those in the 13 regular annual appropriations acts. Supplemental appropriations provide additional budget authority beyond the original estimates for programs or activities (including new programs authorized after the date of the original appropriation act) in cases where the need for funds is too urgent to be postponed until enactment of the next regular appropriation bill. (See Appropriation Act.)
[The Congressional Budget Process: An Explanation, Appendix J (Glossary), Committee on the Budget of the U.S. Senate, S. Prt. 105-67 (Revised December 1998).]