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.