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