noun
millimicron
A unit of length equal to one thousandth of a micron (one nanometer). Now largely replaced by 'nanometer' (ナノメートル) in modern scientific usage.
from French
このフィルターはミリミクロン単位の粒子を除去できる。
This filter can remove particles on the scale of millimicrons.
Borrowed from French 'millimicron', combining 'milli-' (thousandth) and 'micron'.