Definition
The payroll taxes that fund Social Security and Medicare, withheld from every paycheck: 6.2% for Social Security (up to an annual wage cap) plus 1.45% for Medicare, 7.65% total. Your employer pays a matching 7.65% on top; self-employed people pay both halves as self-employment tax.
Why it matters
FICA is why take-home pay drops before income tax even enters the picture, and it applies from the first dollar, with no standard deduction. It is also why net pay jumps mid-year for high earners: Social Security withholding stops once wages pass the annual cap.