Femto-

Femto (symbol f) is a unit prefix in the metric system denoting a factor of 10⁻¹⁵. It was added to the International System of Units (SI) in 1964.[1] It is derived from the Danish word femten, meaning "fifteen", which has a vaguely similar spelling to fermi, which was introduced earlier to mean a femtometer.

Examples of use:

The HIV-1 virus has the mass of about 1 x 10⁻¹⁵ g or 1 fg.
A proton has a diameter of about 1.66 femtometres.
More examples are available here.

The femtometre shares the unit symbol (fm) with the older non-SI unit fermi, to which it is equivalent. The fermi, named in honour of Enrico Fermi, is often encountered in nuclear physics.

SI prefixes
Prefix		Base 10	Decimal	English word		Adoption[nb 1]	Etymology
Name	Symbol	Base 10	Decimal	Short scale	Long scale	Adoption[nb 1]	Language	Source word
yotta	Y	10²⁴	1000000000000000000000000	septillion	quadrillion	1991	Latin	eight[nb 2]
zetta	Z	10²¹	1000000000000000000000	sextillion	trilliard	1991	Latin	seven[nb 2]
exa	E	10¹⁸	1000000000000000000	quintillion	trillion	1975	Greek	six
peta	P	10¹⁵	1000000000000000	quadrillion	billiard	1975	Greek	five[nb 2]
tera	T	10¹²	1000000000000	trillion	billion	1960	Greek	four,[nb 2] monster
giga	G	10⁹	1000000000	billion	milliard	1960	Greek	giant
mega	M	10⁶	1000000	million		1873	Greek	great
kilo	k	10³	1000	thousand		1795	Greek	thousand
hecto	h	10²	100	hundred		1795	Greek	hundred
deca	da	10¹	10	ten		1795	Greek	ten
		10⁰	1	one		–
deci	d	10⁻¹	0.1	tenth		1795	Latin	ten
centi	c	10⁻²	0.01	hundredth		1795	Latin	hundred
milli	m	10⁻³	0.001	thousandth		1795	Latin	thousand
micro	μ	10⁻⁶	0.000001	millionth		1873	Greek	small
nano	n	10⁻⁹	0.000000001	billionth	milliardth	1960	Greek	dwarf
pico	p	10⁻¹²	0.000000000001	trillionth	billionth	1960	Spanish	peak, a little bit
femto	f	10⁻¹⁵	0.000000000000001	quadrillionth	billiardth	1964	Danish	fifteen, Fermi[nb 3]
atto	a	10⁻¹⁸	0.000000000000000001	quintillionth	trillionth	1964	Danish	eighteen
zepto	z	10⁻²¹	0.000000000000000000001	sextillionth	trilliardth	1991	Latin	seven[nb 2]
yocto	y	10⁻²⁴	0.000000000000000000000001	septillionth	quadrillionth	1991	Latin	eight[nb 2]
Prefixes adopted before 1960 already existed before SI. The introduction of the CGS system was in 1873. Part of the beginning of the prefix was modified from the word it was derived from, ex: "peta" (prefix) vs "penta" (source word). The fermi was introduced earlier with the symbol "fm", which prompted the reinterpretation of the "f" as a prefix to "m", with femto being derived from the Danish word femten due to its similarity.

References

"Resolution 8 of the 12th CGPM". International Bureau of Weights and Measures. 1964. Retrieved 2020-05-14.

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[2] Prefixes adopted before 1960 already existed before SI. The introduction of the CGS system was in 1873.

[similar-3] Part of the beginning of the prefix was modified from the word it was derived from, ex: "peta" (prefix) vs "penta" (source word).

[4] The fermi was introduced earlier with the symbol "fm", which prompted the reinterpretation of the "f" as a prefix to "m", with femto being derived from the Danish word femten due to its similarity.

[1] "Resolution 8 of the 12th CGPM". International Bureau of Weights and Measures. 1964. Retrieved 2020-05-14.