What is Budget?

Legal Definition
A budget is a quantitative expression of a financial plan for a defined period of time. It may include planned sales volumes and revenues, resource quantities, costs and expenses, assets, liabilities and cash flows. It expresses strategic plans of business units, organizations, activities or events in measurable terms.

A budget is the sum of money allocated for a particular purpose and the summary of intended expenditures along with proposals for how to meet them.
-- Wikipedia
Legal Definition
A name given in England to the statement annually presented to parliament by the chancellor of the exchequer, containing the estimates of the national revenue and expenditure.
-- Black's Law Dictionary