There has been various definitions given for ushnokdaka (hot/warm water)
Water boiled and reduced to one-eighth, one-fourth or half of the original quantity or mere boiling it is called ushnodaka.
Warm water has various benefits- cures jwara/fever, kaasa/cough, Shwasa/dyspnoea, (aggravated) tridosha, ama and meda/fat; is a digestive and is always healthy.
Acharya susruta says that boiled water which becomes cool after the vessel is covered with a lid (cool by its own accord) mitigates all the three dosas, neither produces dryness nor more moisture inside body, mitigates worms, thirst and fever and is easily digestable; boiled water made cool by pouring it in a stream is constipating and that cooled by air (fanning) is difficult for digestion.
Hot water consumed at nights, breaks the masses of kapha, expels flatus, and digests the undigested food matter quickly.
Water boiled during day, and consumed at night and boiled during night and consumed in the day, becomes hard to digest. Boiled water which has become stale (after a lapse of one day) is devoid of all the qualities or the effects of fire, aggravates all three dosas, is hard for digestion, sour after digestion, and unfit for use in all diseases.
Ref: Bhavaprakhasa