Frost was a layer of frozen water that accumulated as the temperature dropped overnight. It could also build up on the sides of a container filled with cold liquid.
In 2266, Hikaru Sulu reported that frost was building up on the surface of Alfa 177. (TOS: "The Enemy Within")
In 2373, Quark recommended a frosty Bajoran ale to Kira Nerys. (DS9: "Ties of Blood and Water")