a thick cloud of tiny water droplets suspended in the atmosphere at or near the earth's surface which obscures or restricts visibility (to a greater extent than mist; strictly, reducing visibility to below 1 km)

the collision occurred in thick fog

a state or cause of perplexity or confusion

the coffee helped clear the fog in my brain

(with reference to a glass surface) cover or become covered with steam

hot steam drifted about her, fogging up the window

the windscreen was starting to fog up

bewilder or puzzle

she stared at him, confusion fogging her brain

spray with an insecticide

the grass which grows in a field after a crop of hay has been taken