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A new SI prefix for the submultiple 10−21. The symbol for zepto is z. So 1 zm = 10−21 m.

The name is an alteration of Latin sept- 'seven', because it's the seventh multiple of 1000 downwards. It and yocto for 10−24, and their large counterparts zetta and yotta, were added to the SI by the nineteenth CGPM in 1991.

For a full list see Standard SI prefixes.

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