Milk

 was a substance secreted by mammals to feed their young.

Milk-producing species
Whether the female of a species exuded milk from its nipples or from simple ducts to nurture its young was one of the defining traits of mammals. (PROSE: The Year of Intelligent Tigers) Similar nourishing substances were produced by non-mammalian species. Malmooth could drink their own internal milk, (TV: Utopia) while the green milk produced by Skarasens was fed on by the Zygons. (PROSE: Sting of the Zygons, PROSE: The Bodysnatchers)

The only known cure for spectrox toxaemia was the milk from a queen bat. (TV: The Caves of Androzani)

The seemingly male Sontarans could be genespliced to produce milk. (TV: A Good Man Goes to War)

Milk as a beverage
Milk was also used by other species as a drink. The humans of Earth harvested and drank the milk of cows. Milk was often sold in glass bottles, and delivered by milkmen. (TV: The Stolen Earth, Hide)

Milk was often used in tea and coffee to add taste. (PROSE: Theatre of War) Milk was sold in cartons or glass bottles, and delivered by milkmen. (TV: The Stolen Earth) It was sometimes drunk warm as a sleep aid. (TV: Invasion of the Dinosaurs)

In some locations, such as the planet Sto, milk was sold in a Milk Market. (TV: Voyage of the Damned)

The Ninth Doctor travelled to Earth at least every five days, which Rose Tyler attributed to him having to get milk. He seemed annoyed that, of all the universe, it had to come from a cow. (TV: The Empty Child)

The Eleventh Doctor once drank milk from a bottle. (TV: Hide)

Minor references

 * The Tenth Doctor bought some milk. (COMIC: Wrath of the Warrior)
 * The Doctor later tried to get some more milk, but instead ended up at the Whispering Gallery. (COMIC: The Whispering Gallery)
 * Larry Nightingale went out to buy milk. (TV: Blink)
 * Rose commented that they returned to Earth whenever they needed milk. The Ninth Doctor asked why, out of all the species in all the universe, it had to come out of a cow. (TV: The Empty Child)
 * Rory Williams was buying milk when he came across an alien. (COMIC: Rory's Story)
 * Speaking to the ganger version of baby Melody Pond, the Eleventh Doctor told her she should call Amy 'Mummy', not 'Big milk thing'. (TV: A Good Man Goes to War)