1 : many small drops of water floating in the air above the ground, the sea, etc.
[noncount]
Heavy fog made it difficult to see the road.
a patch/blanket of fog
ocean fog
[count]
a climate marked by heavy fogs
The fog reduced visibility to a quarter of a mile.
2 [singular] : a state of mental confusion
This problem has me in a fog.
an alcohol-induced fog