Cyclopedia of Congressional Budget Law
A budgetary lockbox is a procedure by which spending reductions in an area or on a program may be assured to be dedicated to deficit reduction rather than used in some fashion to offset or be directed toward some other purpose. Lockboxes have taken on various legislative forms and been incorporated into various bills. The term is somewhat loosely defined in that disparate procedures might be termed as such, but operate in different ways.
Two examples of lockboxes might be Spending Reduction Accounts in the House and the lock box mechanism incorporated into the Line Item Veto Act (section 1021 of Part C of the Impoundment Control Act of 1974).
GAO Glossary of Terms and Definition (September 2005)
In the budget context, any of several legislative mechanisms that attempt to isolate, or “lock away,” funds of the federal government for purposes such as reducing federal spending, preserving surpluses, or protecting the solvency of trust funds.