Final answer:
An uncontrollable expenditure is one that is legally required due to existing laws or obligations, typically associated with mandatory spending on programs like Social Security.
Step-by-step explanation:
An "uncontrollable" expenditure in the federal budget is defined as an expenditure that is required by current law or a previous government obligation to people automatically eligible for some benefit. These are often related to mandatory spending on entitlement programs that individuals are eligible for if they meet the requirements. Since mandatory spending is legally obligated, such as Social Security and Medicare, it's not easily adjusted in the budget, distinguishing it from discretionary spending that can be altered by Congress during the budgeting process.