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zepto-

In SI, the decimal submultiplier prefix that signifies 10⁻²¹, or 1 sextillionth (American system) of the unit modified (1000 trillionth, continental system). Symbol, z.

The name is derived from “septo,” from the Latin for seven, because the prefix represents 10⁻³ to the seventh power. Since “s” was already a symbol for a unit (the second), the symbol for a septosecond would have been “ss”. To avoid this typo-prone outcome, “z” was substituted for “s”. Hence “zepto.”

The prefix was proposed by the CIPM in 1990, and adopted by the 19ᵗʰ CGPM in 1991. Compare zetta-.

Comptes Rendus des Sèances 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 13, page 40.