A budget is an estimate, a calculation of how much a person, family or country should receive and how much to spend in a given period. For example, the household budget. To calculate it, you need to consider the salary of family members living in the same household and how much they should spend on food, plus the rent on the house, plus the school fees of the children, plus the family car fuel, or bus ticket expenses, etc. The same goes for the public budget. To run a country, every government needs to predict how much money it will spend. Based on this estimate, the government defines spending on health, education, security, etc. All these calculations form the public budget.
Other word suggestions…
Parliamentary Committee of Inquiry (CPI)
Treaty of Tordesillas