An entitlement is a federal program or provision of law that requires payments to any person or state or local government that meets the eligibility criteria established by law. Entitlements constitute a binding obligation on the part of the federal government, and eligible recipients have legal recourse if the obligation is not fulfilled. Social Security, veterans' compensation, and government pensions are examples of entitlement programs.
The Congressional Budget Act of 1974 attempted to limit the use of entitlement authority. The key characteristic of entitlements is that, once enacted, the authorizing legislation automatically creates enforceable claims to benefits and thus effectively preempts the formal appropriations process. Thus, the determination of outlays at any given time is a function of variables outside the control of the appropriations committees.
In 2013, 62% of the United States federal budget went for non-discretionary, mandatory, entitlement spending.
-  US Senate Reference