Your budget is the amount of money that you have available to spend. The budget for something is the amount of money that a person, organization, or country has available to spend on it.
The budget of an organization or country is its financial situation, considered as the difference between the money it receives and the money it spends.
In Britain, the Budget is the financial plan in which the government states how much money it intends to raise through taxes and how it intends to spend it. The Budget is also the speech in which this plan is announced.