I am confused about mils, mm and microns

When poly is referred to as 4 mil does this mean 4 thousanths of an inch
If this is the case then would not 40 mil poly be 1 millimeter thick
(1mm=40 thou) ??
Or when you speak of mil do you mean 100 micron thick poly?
Sorry to ask so basic a question but thanks for helping out.
Kind regards,