A glass of hot milk with nutmeg.
Milk is a creamy white liquid produced from the mammary glands of several mammalian species, including the Earth cow and the Klingon targ. (TNG: "In Theory") On Earth, it is often consumed as a nutritious beverage. Milk can also be processed into dairy products such as cheese and ice cream.
Widin Dairy Farm delivered milk via a horse-drawn cart to the inhabitants of New York City in 1930. (TOS: "The City on the Edge of Forever")
2% milk was one of several beverages offered in the drink dispensers in the mess halls of NX-class starships. (ENT: "Terra Prime")
Lt. Kevin Riley drank milk while on duty in the engineering section of the USS Enterprise before it was poisoned by Lenore Karidian in 2266. (TOS: "The Conscience of the King")
When milk is heated, the lactose it contains acts as a mild sedative, helping one sleep. Warm milk was Lt. Reginald Barclay's drink of choice on the USS Enterprise. (TNG: "Hollow Pursuits") Captain Picard's aunt Adele had a recipe for steamed milk with nutmeg, which Picard shared with Doctor Crusher. (TNG: "Cause and Effect") On two separate occasions, Dr. Crusher prescribed a warm glass of milk to William Riker (TNG: "Schisms") and Jean-Luc Picard (with a dash of nutmeg) (TNG: "All Good Things...") to help them sleep.
Quark prepared warm enyak's milk when Kira Nerys appeared "rumpled." (DS9: "Ties of Blood and Water") Similarily, Q suggested warm Kylerian goat milk to Captain Kathryn Janeway when she was having trouble sleeping. (VOY: "Death Wish")
Geordi La Forge compared the spaceborne species dubbed Junior feeding on the energy of the USS Enterprise-D to an infant being fed with milk. In order to dispose of the creature which was rapidly draining the ship's power, he realized that he had to "sour the milk". (TNG: "Galaxy's Child")
On some replicators, a milk button was available for easy manual input.
Both Commander Chakotay and Neelix thought that warm milk was repulsive. (VOY: "Unforgettable")
- This was shown in the It's A Wrap! sale and auction, which features a close-up of a replicator's manual control interface, which included a button for 'milk'.