I was referring to the basic rules of VFR cloud clearances and visibility, which change based on the airspace you are in. You know, like 91.155.Do you have an example of the FAA using it that way?
As already pointed out, the FAA defines "instrument meteorological conditions (IMC)" as "Meteorological conditions expressed in terms of visibility, distance from cloud, and ceiling less than the minima specified for visual meteorological conditions."
So if you are in conditions less than 91.155 minima, you are in IMC and prohibited from flying in that airspace.