femtosecond n. 10 to the -15th power or 0.000000000000001 second, equalling a quadrillionth of a second or 0.3 light microns. Editorial Note: Commonly used in measurement of light in laser or light-related technologies. Etymological Note: Danish or Norwegian femt, from femten “fifteen” + -o. Femto was established as a standard prefix equalling 10 to the -15th power by the International Union of Pure and Applied Physics in 1961. (source: Double-Tongued Dictionary)

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