Atto-

Atto (symbol a) is a unit prefix in the metric system denoting a factor of 10⁻¹⁸ or 0.000000000000000001.

The unit multiple was adopted at the 12th General Conference on Weights and Measures (CGPM) in Resolution 8.^[1] It is derived from the Danish word atten, meaning "eighteen".

Examples of its use:

Weight of the HIV-1 virus is about 1×10⁻¹⁸ kg or 1×10⁻¹⁵ g, which can be written as 1 fg or 1000 ag.
More examples available.

SI prefixesv · d · e
Prefix		Base 10	Decimal	English word		Adoption^{[nb 1]}
Name	Symbol	Base 10	Decimal	Short scale	Long scale	Adoption^{[nb 1]}
yotta	Y	10²⁴	1000000000000000000000000	septillion	quadrillion	1991
zetta	Z	10²¹	1000000000000000000000	sextillion	trilliard	1991
exa	E	10¹⁸	1000000000000000000	quintillion	trillion	1975
peta	P	10¹⁵	1000000000000000	quadrillion	billiard	1975
tera	T	10¹²	1000000000000	trillion	billion	1960
giga	G	10⁹	1000000000	billion	milliard	1960
mega	M	10⁶	1000000	million		1873
kilo	k	10³	1000	thousand		1795
hecto	h	10²	100	hundred		1795
deca	da	10¹	10	ten		1795
		10⁰	1	one		–
deci	d	10⁻¹	0.1	tenth		1795
centi	c	10⁻²	0.01	hundredth		1795
milli	m	10⁻³	0.001	thousandth		1795
micro	μ	10⁻⁶	0.000001	millionth		1873
nano	n	10⁻⁹	0.000000001	billionth	milliardth	1960
pico	p	10⁻¹²	0.000000000001	trillionth	billionth	1960
femto	f	10⁻¹⁵	0.000000000000001	quadrillionth	billiardth	1964
atto	a	10⁻¹⁸	0.000000000000000001	quintillionth	trillionth	1964
zepto	z	10⁻²¹	0.000000000000000000001	sextillionth	trilliardth	1991
yocto	y	10⁻²⁴	0.000000000000000000000001	septillionth	quadrillionth	1991
↑ Prefixes adopted before 1960 already existed before SI. 1873 was the introduction of the CGS system.

References

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