When there is fog, there are tiny drops of water in the air which form a thick cloud and make it difficult to see things.

A fog is an unpleasant cloud of something such as smoke inside a building or room.

You can use fog to refer to a situation which stops people from being able to notice things, understand things, or think clearly.

If a window, mirror, or other glass surface fogs or is fogged, it becomes covered with very small drops of water so that you cannot see things clearly through it or in it.