The payroll tax that finances Social Security and Medicare in the United States is FICA.
FICA stands for the Federal Insurance Contributions Act, the 1935 law that established payroll contributions for Social Security. Medicare payroll taxation was added later, in 1965, when the federal health-insurance program for older Americans was created. Employers generally withhold the employee share from wages and pay a matching employer share.
FICA is not the same as federal income tax. Income tax withholding helps fund the federal government generally, while FICA is associated with specific social-insurance programs. The Social Security and Medicare portions are also calculated under different rules.
The Social Security portion is subject to an annual wage base, so wages above that limit are not subject to the Social Security tax in the same way. The Medicare portion has no equivalent wage cap, and an additional Medicare tax can apply to some higher-income employees. Self-employed workers generally pay the combined equivalent through self-employment tax.