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. Strax was very proud that he could produce "magnificent quantities" of lactic fluid and offered to nurse Melody Pond. (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 sold in cartons or glass bottles, and delivered by milkmen. In some locations, such as the planet Sto, milk was sold in a Milk Market. (TV: The Stolen Earth, Hide, Voyage of the Damned)

Milk was often used in tea and coffee to add taste. It was sometimes drunk warm as a sleep aid. (PROSE: Theatre of War, TV: Invasion of the Dinosaurs)

The Doctor
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 Tenth Doctor and Martha fought and then helped a viking warrior they ran into when they stopped to buy milk. Another time they tried to get milk they ended up in the Whispering Gallery instead of a grocers as intended. (COMIC: Wrath of the Warrior, The Whispering Gallery)

Larry Nightingale met the Doctor when he returned from a milk run. (TV: Blink)

Rory Williams ran into trouble when he went to the market to get milk and a whisk for the Eleventh Doctor who was making proper ice cream in the TARDIS. He ended up pretending to be the Doctor in order to get an alien to leave. (COMIC: Rory's Story)

The Doctor later admonished the ganger version of baby Melody Pond, telling her that she should call Amy 'Mummy', not 'Big milk thing'. (TV: A Good Man Goes to War)

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

Milk as an ingredient
Milk was also the base ingredient for many foods, known as dairy foods. The Doctor showed a great fondness for most foodstuffs made from milk, keeping track of which galaxies had the best milkshakes and most expensive ice cream. (COMIC: Agent Provocateur, WC: Escape to Penhaxico)

The Doctor claimed to have a doctorate in cheese making. (TV: The God Complex)

Shortly after regeneration, the Eleventh Doctor tried several foods, among them buttered bread, and yoghurt before settling on custard. (TV: The Eleventh Hour)