Yocto-

Yocto- (symbol y) is a prefix in the metric system denoting a factor of 10⁻²⁴ or 0.000000000000000000000001.

Adopted in 1991 by the General Conference on Weights and Measures, it comes from the Greek οκτώ, meaning "eight", because it is equal to 1000⁻⁸. As of 2009, yocto is the smallest confirmed SI prefix.

It can be used to state a subatomic particle's mass, for example:

Rest mass of electron:	0.000911 yg
Rest mass of proton:	1.6726 yg
Rest mass of neutron:	1.6749 yg

SI prefixes

Prefix	Symbol	1000m	10n	Decimal	Short scale	Long scale	Scientific notation	Since
yotta	Y	10008	1024	1000000000000000000000000	Septillion	Quadrillion	1E24	1991
zetta	Z	10007	1021	1000000000000000000000	Sextillion	Trilliard	1E21	1991
exa	E	10006	1018	1000000000000000000	Quintillion	Trillion	1E18	1975
peta	P	10005	1015	1000000000000000	Quadrillion	Billiard	1E15	1975
tera	T	10004	1012	1000000000000	Trillion	Billion	1E12	1960
giga	G	10003	109	1000000000	Billion	Milliard	1E9	1960
mega	M	10002	106	1000000	Million		1E6	1960
kilo	k	10001	103	1000	Thousand		1E3	1795
hecto	h	10002/3	102	100	Hundred		1E2	1795
deca	da	10001/3	101	10	Ten		1E1	1795
		10000	100	1	One		–	–
deci	d	1000−1/3	10−1	0.1	Tenth		1E-1	1795
centi	c	1000−2/3	10−2	0.01	Hundredth		1E-2	1795
milli	m	1000−1	10−3	0.001	Thousandth		1E-3	1795
micro	μ	1000−2	10−6	0.000001	Millionth		1E-6	1960
nano	n	1000−3	10−9	0.000000001	Billionth	Milliardth	1E-9	1960
pico	p	1000−4	10−12	0.000000000001	Trillionth	Billionth	1E-12	1960
femto	f	1000−5	10−15	0.000000000000001	Quadrillionth	Billiardth	1E-15	1964
atto	a	1000−6	10−18	0.000000000000000001	Quintillionth	Trillionth	1E-18	1964
zepto	z	1000−7	10−21	0.000000000000000000001	Sextillionth	Trilliardth	1E-21	1991
yocto	y	1000−8	10−24	0.000000000000000000000001	Septillionth	Quadrillionth	1E-24	1991