The cycle per second was a once-common unit of frequency.

With the organisation of the International System of Units (abbreviated SI from the French) in 1960, the cycle per second was officially replaced by the hertz, or reciprocal second—i.e. the cycle in 'cycle per 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 per second' all but extinct.

Nonetheless, the change is not without its critics, who argue that the hertz should be redefined as simply a renaming of the cycle per 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 per second has perhaps a better claim than the cycle per second to being equivalent to the reciprocal second. But the hertz is overwhelmingly (some would say exclusively) used in the sense of cycle per second, and certainly never used in the sense of radian per second.

The obsolete terms kilocycle, megacycle and kilomegacycle still occur in some older documents; the modern equivalents are kilohertz, megahertz and gigahertz, respectively.

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