Is an area of localized reduced visibility considered to be a cloud? If there an area containing visible moisture in an otherwise clear sky and that area of visible moisture is transparent is it considered a "thin cloud" or simply a "localized area of reduced visibility"? 

What about areas of mist and haze with no definite border below an overcast layer that present in the sky as large areas of reduced visibility still above VFR visibility limitations for aircraft within them?
This might be an answer that works: In meteorology, the definition of fog is less than 1000 m visiblity and mist/haze more than 1000 m visibility.  I think you could reasonably infer the same definition to apply to higher cloud as well.  To the extent you are looking for a go/no/go definition, I think that might be all there is.