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This article is about the SI prefix. For the X Window System acceleration architecture, see EXA.

Look up exa- in Wiktionary, the free dictionary.

exa (symbol E) is a prefix in the SI system of units denoting 10¹⁸ or 1 000 000 000 000 000 000.

Adopted in 1975, it comes from the Greek ἕξ, meaning six (like hexa), because it is equal to 1000⁶.

Examples:

1 EeV = 10¹⁸ electronvolts = 0.1602 joule
1 exasecond is approximately 32 billion years
1 exametre is approximately 110 light years
0.43 Es ≈ the approximate age of the Universe
1.6 Em — 172 ± 12.5 light years — Diameter of Omega Centauri (one of the largest known globular clusters, perhaps containing over a million stars)^[1]^[2]

SI prefixes
1000^m	10ⁿ	Prefix	Symbol	Since^[1]	Short scale	Long scale	Decimal
1000⁸	10²⁴	yotta	Y	1991	Septillion	Quadrillion	1000000000000000000000000
1000⁷	10²¹	zetta	Z	1991	Sextillion	Trilliard	1000000000000000000000
1000⁶	10¹⁸	exa	E	1975	Quintillion	Trillion	1000000000000000000
1000⁵	10¹⁵	peta	P	1975	Quadrillion	Billiard	1000000000000000
1000⁴	10¹²	tera	T	1960	Trillion	Billion	1000000000000
1000³	10⁹	giga	G	1960	Billion	Milliard	1000000000
1000²	10⁶	mega	M	1960	Million		1000000
1000¹	10³	kilo	k	1795	Thousand		1000
1000^²⁄₃	10²	hecto	h	1795	Hundred		100
1000^¹⁄₃	10¹	deca	da	1795	Ten		10
1000⁰	10⁰	(none)	(none)	NA	One		1
1000^−¹⁄₃	10⁻¹	deci	d	1795	Tenth		0.1
1000^−²⁄₃	10⁻²	centi	c	1795	Hundredth		0.01
1000⁻¹	10⁻³	milli	m	1795	Thousandth		0.001
1000⁻²	10⁻⁶	micro	µ	1960^[2]	Millionth		0.000001
1000⁻³	10⁻⁹	nano	n	1960	Billionth	Milliardth	0.000000001
1000⁻⁴	10⁻¹²	pico	p	1960	Trillionth	Billionth	0.000000000001
1000⁻⁵	10⁻¹⁵	femto	f	1964	Quadrillionth	Billiardth	0.000000000000001
1000⁻⁶	10⁻¹⁸	atto	a	1964	Quintillionth	Trillionth	0.000000000000000001
1000⁻⁷	10⁻²¹	zepto	z	1991	Sextillionth	Trilliardth	0.000000000000000000001
1000⁻⁸	10⁻²⁴	yocto	y	1991	Septillionth	Quadrillionth	0.000000000000000000000001
The metric system was introduced in 1795 with six prefixes. The other dates relate to recognition by a resolution of the CGPM. The 1948 recognition of the micron by the CGPM was abrogated in 1967.

[edit] See also

^ distance × sin( diameter_angle ), using distance of 5kpc (15.8 ± 1.1 kly) and angle 36.3', = 172 ± 12.5 ly.
^ van de Ven, G.; van den Bosch, R. C. E.; Verolme, E. K.; de Zeeuw, P. T. (January II 2006). "The dynamical distance and intrinsic structure of the globular cluster ω Centauri". Astronomy and Astrophysics 445 (2): 513–543. doi:10.1051/0004-6361:20053061. http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2006A%26A...445..513V. "best-fit dynamical distance D=4.8±0.3 kpc ... consistent with the canonical value 5.0±0.2 kpc obtained by photometric methods".