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.