A budget is an account of the money to be spent on a project, or by a person or organization in a time period. It usually lists the various things that are to be done, and how much to spend on each. It may also list estimated revenues. When spending is less than revenues, it is a surplus budget. When spending is more than revenues, it is a deficit budget. When they are the same, it's a balanced budget.
An organization's budget is usually for a fiscal year.