yocto-

Adopted by the Conference General des Poids et Mesures [CGPM] in Paris at its 19th meeting in October, 1991. This prefix is used in the metric [decimal] system as the septillionth part [U.S.] or quintillionth part [U.K.], 10^-24 [0.000 000 000 000 000 000 000 001]. The metric symbol for yocto- is y. "The names yocto and yotta are derived from octo, suggesting the number eight [the eighth power of 10³]; the letter 'y' is added to avoid the use of the letter 'o' as a symbol because it may be confused with the number zero." Bureau International des Poids et Mesures, 1990, document #10, pp. 44-45.