I would say that it is the density of the water particles in the air. Where I come from (UK) mist tends to be thought of as thinner than fog - you can see further in it. Both are formed by water condensing in the air when the temperature drops. Fog is more like a cloud that is at ground level.
You also see mist form in a layer above water, where evaporating water from the surface stays close when the air is still.
If you are interested, wiki has a good summary.