This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

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n. A unit of time equal to 0.000 000 000 000 001 seconds and with symbol fs.

n. one quadrillionth (10^-15) of a second; one thousandth of a picosecond

"One femtosecond is 10-15 seconds, that is, 0.000000000000001 seconds, which is to a second as a second is to 32 million years."

time unit    unit of time   

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