Atto- (symbol a) is an SI prefix to a unit and means that it is 10^-18 times this unit. Examples are one attosecond or one attometre (U.S. spelling attometer).

Adopted in 1964, it comes from the Danish atten, meaning eighteen.

SI prefixes

10n	Prefix	Symbol	Short scale	Long scale Name	Decimal Equivalent
1024	yotta	Y	Septillion	Quadrillion	1 000 000 000 000 000 000 000 000
1021	zetta	Z	Sextillion	Trilliard (thousand trillion)	1 000 000 000 000 000 000 000
1018	exa	E	Quintillion	Trillion	1 000 000 000 000 000 000
1015	peta	P	Quadrillion	Billiard (thousand billion)	1 000 000 000 000 000
1012	tera	T	Trillion	Billion	1 000 000 000 000
109	giga	G	Billion	Milliard (thousand million)	1 000 000 000
106	mega	M	Million		1 000 000
103	kilo	k	Thousand		1 000
102	hecto	h	Hundred		100
101	deca, deka	da	Ten		10
10−1	deci	d	Tenth		0.1
10−2	centi	c	Hundredth		0.01
10−3	milli	m	Thousandth		0.001
10−6	micro	µ	Millionth		0.000 001
10−9	nano	n	Billionth	Milliardth	0.000 000 001
10−12	pico	p	Trillionth	Billionth	0.000 000 000 001
10−15	femto	f	Quadrillionth	Billiardth	0.000 000 000 000 001
10−18	atto	a	Quintillionth	Trillionth	0.000 000 000 000 000 001
10−21	zepto	z	Sextillionth	Trilliardth	0.000 000 000 000 000 000 001
10−24	yocto	y	Septillionth	Quadrillionth	0.000 000 000 000 000 000 000 001
edit  (http://www.wordiq.com/definition/Template:SI_prefixes)

See also

• SI prefix
• attometre
• attosecond

ca:Atto es:Atto fr:Atto sl:Ato