A once-common unit of frequency. Sine waves of various frequencies; the lower waves have higher frequencies than those above. ...

With the organisation of SI in 1960, the cycle/second was officially replaced by the Hertz, or reciprocal second---i.e. the cycle in cycle/second was dropped. Perhaps because of the convenient brevity it brings to both speech and writing, this particular mandate has been so widely adopted as to render the old cycle/second all but extinct. Cover of brochure The International System of Units. ... The hertz (symbol: Hz) is the SI unit of frequency. ...

Nonetheless, the change is not without its critics, who argue that the Hertz should be redefined as simply a renaming of the cycle/second it displaced, rather than as the reciprocal second. Since SI has adopted the radian as its preferred but dimensionless unit of angle, but left its explicit writing discretionary in unit combinations, the radian/second has perhaps a better claim than the cycle/second to being equivalent to the reciprocal second. But the Hertz is overwhelmingly (some would say exclusively) used in the sense of cycle/second, and certainly never used in the sense of radian/second. The radian (symbol: rad, or a superscript c ( half circle)) is the SI unit of plane angle. ...