social security tax - in U.S.A., the equivalent of the British national insurance contributions, paid by employer and employee and by the self-employed, to provide social security benefits to unemployed, sick, disabled and other groups of persons unable to work, and retirement pensions in old age, riiklikuks pensioni- ja ravikindlustuseks vajaliku tulu saamiseks maksumaksjale pandud rahaline kohustis, mis kuulub täitmisele käesolevas seaduses ettenähtud korras, suuruses ja tähtaegadel