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In units of measurement, a factor of 10-15.

Danish or Norwegian femten, fifteen.

This is one of the standard SI (Système International) decimal prefixes (see the entry Number words), which occurs in compounds such as femtogram. Since chemical reactions take place in periods of time measured in femtoseconds (one femtosecond is to a second as a second is to 32 million years), a speciality called femtochemistry has grown up using techniques such as ultrafast laser pulses to observe their progress directly.

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