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A femtohertz is a SI prefixes applied unit of frequency, symbol fHz, is a multiple of the of SI prefixes (femto) used with the hertz.
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., c/s, ~, or, ambiguously, just cycles. The term comes from the fact that sound waves have a frequency measurable in their number of vibrations, or cycles, per second.