# Cycle per second

Crystal resonators, the 1940s-era center unit is marked in kc.

The cycle per second was a once-common English name for the unit of frequency now known as the hertz. The plural form was typically used, often written "cycles per second," "cycles/second", "c.p.s," "~" or just "cycles."

With the organization 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. Symbolically, "cycle per second" units are "cycle/second", while hertz is "1/second" or $\text{s}^{-1}$.[1] This particular mandate has been so widely adopted as to render the old 'cycle per second' all but extinct.

For higher frequencies, the terms kilocycle (kc), megacycle (Mc) and (less commonly) kilomegacycle (kMc) were used before 1960[2] and in some later documents.[3] The modern equivalents are kilohertz (kHz) and megahertz (MHz). Higher orders of magnitude are represented in gigahertz (GHz), terahertz (THz), petahertz (PHz), and exahertz (EHz).

The rate at which aperiodic or stochastic events (such as radioactive decay) occur is expressed in becquerels, not hertz, since although the two are mathematically similar by convention hertz implies regularity where becquerels implies the requirement of a time averaging operation. Thus 1 Bq is 1 event per second on average whereas 1 Hz is 1 event per second in a regular cycle.