The kind of government program that provides individuals with personal financial benefits to which an indefinite (but usually rather large) number of potential beneficiaries have a legal right (enforceable in court, if necessary) whenever they meet eligibility conditions that are specified by the standing law that authorizes the program. The most important examples of entitlement programs at the federal level in the United States would include Social Security, Medicare, and Medicaid.
Sentence
Over half of the budget is spent on mandatory entitlement spending, with over $944,143,000,000 spent on Social Security in 2016.