An arbitrary alteration of classical Latinsepti-septi-comb. form1 (compare ‑o‑connective); so called with reference to the unit being seven orders of magnitude smaller than one thousandth (10−3). Compare yocto-comb. form, and…
The unit prefix was officially accepted by the General Conference on Weights and Measures in 1991. On the further background compare:
1994
In 1990 the International Union of Pure and Applied Chemistry..offered for discussion a suggestion for sepo and oco, from Latin septo, seven and octo, eight. Because the symbol s for sepo would be the same as the symbol for the second, while the symbol o for oco could be misread as a zero, these eventually became zepto (10−21) and yocto (10−24).
Quarterly Journal Royal Astron. Societyvol. 25 570
Meaning & use
Prefixed to the names of units of measurement to denote a factor of 10−21 (one thousand million million millionth; one thousandth to the power of seven), as zeptogram, zeptosecond, etc. Symbol z. Also: forming corresponding adjectives, as zeptomolar.
1991
After further discussion along this line the CCU decided as follows:..10−21 zepto [symbol] z... 10−24 yocto [symbol] y.
Rapport de la 10e Session, Commité Consultatif des Uniés (Bureau Internat. des Poids et Mesures) u44
1994
The elementary charge is 1.6 x 10−19 coulombs (160 zeptocoloumbs) and the mass of a proton is 1.7 yoctograms.
New Scientist 3 December 81/2
2000
A unit density particle of 2nm diameter weighs a zeptogram.
Adv. Chromatrographyvol. 40 281
2001
The signal is then detected (limit in the low zeptomolar range) by the two detectors of the Odyssey IR Imager.
Scientist (Nexis) 14 May 20
2011
Even bandwidth-limited x-ray pulses are not sufficiently short to resolve motion within nuclei that occurs even faster on the time scale of zeptoseconds.
S. P. Hau-Riege, High-intensity X-rays vi. 159
Pronunciation
Stress is usually determined by a subsequent element and vowels may be reduced accordingly. Some degree of stress is usually maintained on the first syllable, e.g. zeptosecond
British English
/ˈzɛptə(ʊ)ˌsɛknd/
ZEP-toh-seck-uhnd
U.S. English
/ˈzɛptoʊˌsɛk(ə)n(d)/
ZEP-toh-seck-uhnd
Consonants
ppea
ttea
kkey
bbuy
ddye
ɡguy
tʃchore
dʒjay
ffore
θthaw
ssore
ʃshore
vvee
ðthee
zzee
ʒbeige
xloch
hhay
llay
ɬrhingyll
rray
wway
jyore
mmay
nnay
ŋsing
Some consonants can take the function of the vowel in unstressed syllables. Where necessary, a syllabic marker diacritic is used, hence /ˈpɛtl/ but /ˈpɛtl̩i/.
Vowels
iːfleece
ihappy
ɪkit
ɛdress
atrap, bath
ɑːstart, palm, bath
ɒlot
ɔːthought, force
ʌstrut
ʊfoot
uːgoose
əletter
əːnurse
ɪənear
ɛːsquare
ʊəcure
eɪface
ʌɪpride
aʊmouth
əʊgoat
ɔɪvoice
ãgratin
ɒ̃salon
ᵻ(/ɪ/-/ə/)
ᵿ(/ʊ/-/ə/)
Other symbols
The symbol ˈ at the beginning of a syllable indicates that that syllable is pronounced with primary stress.
The symbol ˌ at the beginning of a syllable indicates that that syllable is pronounced with secondary stress.
Round brackets ( ) in a transcription indicate that the symbol within the brackets is optional.
Some consonants can take the function of the vowel in unstressed syllables. Where necessary, a syllabic marker diacritic is used, hence /ˈpɛd(ə)l/ but /ˈpɛdl̩i/.
Vowels
ifleece, happy
ɪkit
ɛdress
ætrap, bath
ɑlot, palm, cloth, thought
ɑrstart
ɔcloth, thought
ɔrnorth, force
ʊfoot
ugoose
əstrut, comma
ərnurse, letter
ɪ(ə)rnear
ɛ(ə)rsquare
ʊ(ə)rcure
eɪface
aɪpride
aʊmouth
oʊgoat
ɔɪvoice
ɑ̃gratin
æ̃salon
ᵻ(/ɪ/-/ə/)
ᵿ(/ʊ/-/ə/)
Other symbols
The symbol ˈ at the beginning of a syllable indicates that that syllable is pronounced with primary stress.
The symbol ˌ at the beginning of a syllable indicates that that syllable is pronounced with secondary stress.
Round brackets ( ) in a transcription indicate that the symbol within the brackets is optional.
Simple text respell breaks words into syllables, separated by a hyphen. The syllable which carries the primary stress is written in capital letters. This key covers both British and U.S. English Simple Text Respell.
Consonants
b, d, f, h, k, l, m, n, p, r, s, t, v, w and z have their standard English values
gguy
jjay
yyore
chchore
khloch
shshore
ththaw
dhthee
zhbeige
Vowels
atrap
ahpalm
airsquare
arstart
arrcarry (British only)
awthought
ayface
a(ng)gratin
edress
eefleece
eerdeer
errmerry
ikit
ighpride
irrmirror
olot (British only)
ohgoat
oogoose
oorcure
orforce
orrsorry (British only)
owmouth
oyvoice
o(ng)salon
ustrut
uhletter
urnurse
urrhurry
uufoot
zepto-, comb. form was first published in September 2016.
zepto-, comb. form was last modified in December 2024.
British English
U.S. English