
Cycle per second

The cycle per second was a once-common English name for the unit of frequency now known as the hertz (Hz). The plural form was typically used, often written cycles per second, cycles/second, c.p.s., c/s, ~, or, ambiguously, just cycles (Cy./Cyc.). The term comes from the fact that sound waves have a frequency measurable in their number of oscillations, or "cycles", per second. 
With the organization of the International System of Units in 1960, the cycle per second was officially replaced by the hertz, or reciprocal second, "s−1" or "1/s". Symbolically, "cycle per second" units are "cycle/second", while hertz is "Hz" or "s−1". Confusion can arise because the units of angular measure (cycle or radian) are sometimes omitted in SI. For higher frequencies, "kilocycles" (kc), as an abbreviation of "kilocycles per second" were often used on components or devices. Other higher units like "megacycle" (Mc) and less commonly "kilomegacycle" (kMc) were used before 1960
and in some later documents. These have modern equivalents such as kilohertz (kHz), megahertz (MHz), and gigahertz (GHz). Following the introduction of the SI standard, use of these terms began to fall off in favor of the new unit, with hertz becoming the dominant convention in both academic and colloquial speech by the 1970s.
The rate at which aperiodic or stochastic events occur may be expressed in becquerels (as in the case of radioactive decay), 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, one becquerel is one event per second "on average", whereas one hertz is one event per second on a regular cycle.
Cycle can also be a unit for measuring usage of reciprocating machines, especially presses, in which cases "cycle" refers to one complete revolution of the mechanism being measured (i.e. the shaft of a reciprocating engine).
Derived units include cycles per day (cpd) and cycles per year (cpy).