VM, it used to be defined in a past company in Switzerland (where they get a lot of it) as fog which freezes on to exposed surfaces. The supposition was that the surfaces were at ambient temp. and that the fog was composed of unstable super cooled droplets (ie below 0 deg) ,waiting for a contact to solidify them.
Trouble is, as you know, you can have ice formation from fog well over 0 deg., if the airframe is cold. Cold soaked tankered fuel in wing tanks can attract ice even over 10 deg OAT if the air is damp enough. The Hold Over Table is a pretty good guide of when to expect the traditional stuff but one peek is worth a thousand tables!