In SI, the decimal submultiplier prefix indicating 10⁻²⁴, a septillionth of the unit to which it is prefixed. Symbol, y. The name comes from “octo,” Latin for eight, because the prefix represents the eighth power of 10⁻³. The “y” was added to avoid using the letter “o” as a symbol, because it might be confused with the numeral for zero.

The yocto- was proposed by the CIPM in 1990, and approved by the 19th CGPM in September-October 1991. Compare yotta-.

Comptes Rendus des Séances de la Conférence Generale des Poids et Mesures, 185.

T. J. Quinn.
News from the BIPM.
Metrologia, volume 29, no. 1, page 3 (1992).

Barry N. Taylor, editor.
The International System of Units.
NIST Special Publication 330, 1991 edition.
Washington, DC: U.S.G.P.O. 1991.

See footnote 14, page 40.

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