yocto-

prefix

Etymology: Ancient Greek ὀκτώ (oktṓ, “eight”), for the eighth order of 10⁻³. The letter y was added, as the second term in a series running backwards through the alphabet.

1. In the International System of Units and other metric systems of units, multiplying the unit to which it is attached by 10⁻²⁴ short scale septillionth or long scale quadrillionth).