according to wikipedia
Fog is distinct from mist only in its density. Fog is defined as cloud which reduces visibility to less than 1 km, whereas mist is that which reduces visibility to less than 2 km.
The BBC weather website confirms that the 'official' definition of fog requires visibility to be less than 1km but it points out that such a definition is more appropriate for aviation purposes than for, say, driving. The BBC site suggests that a 200m figure is more appropriate for the general public. (That certainly seems to fit in with the terms used in their weather forecasts): http://www.bbc.co.uk/weather/features/understa nding/fog.shtml