A new SI prefix for the submultiple 10−24. The symbol for yocto is y. So 1 ym = 10−24 m.

The name is an alteration of Latin oct- 'eight', because it's the eighth multiple of 1000 downwards. It and zepto for 10−21, and their large counterparts yotta and zetta, were added to the SI by the nineteenth CGPM in 1991.

For a full list see Standard SI prefixes.

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