Fog or mist is a concentrated suspension of water vapor, and is formed when water condenses into tiny liquid water droplets in the air.
At the Battle of Valcour Island during the American Revolution, the Colonial fleet was able to escape Valcour Bay and avoid the Royal Navy's notice due to the arrival of fog from the north. (Enterprise Logs short story: "The Veil at Valcour")
When the USS Enterprise approached the planet K-G, it passed through a "space fog" containing plant spores which fastened to the hull and penetrated the ship. (TOS comic: "The Planet of No Return")
The planet Dantus III was perpetually shrouded in fog. (DS9 novel: The Siege) The planet Tellar had soft, seasonless mists. (TOS novel: Prime Directive)