(covers information from several alternate timelines)
Kirk and Spock slipping on ice
Lieutenant Torres frozen by Q
Ice is the solid form of water, formed at temperatures below 0 °C. Comets consist mostly of ice. (TNG: "Masks"; ENT: "Breaking the Ice")
In 2270, an entire deck of the USS Enterprise was covered with ice. James T. Kirk and Spock slipped on it. (TAS: "The Practical Joker")
Ice can be used for medicinal purposes, notably to reduce swelling. (VOY: "Heroes and Demons"; DS9: "Badda-Bing, Badda-Bang")
It can also be used for cooling drinks such as martinis and champagne. (TOS: "The Cage"; TNG: "We'll Always Have Paris")
An icicle is a spike of ice formed from the freezing and thawing of falling water. While being held prisoner in the freezing cold rec room by the Enterprise main computer, Leonard McCoy suggested that he, Hikaru Sulu and Nyota Uhura move before they turned into icicles. (TAS: "The Practical Joker") Intendant Kira once told Benjamin Sisko that she knew of Breen icicles warmer than Jennifer Sisko. (DS9: "Shattered Mirror")
The entity Q used his powers and "iced" first Lieutenant Torres and later Natasha Yar. Both were recovered a short time later. (TNG: "Encounter at Farpoint")