The nutrient-rich liquid from the mammary glands of mammals. Common commercial or farmed dairy sources are cow, sheep, goat, water buffalo, and camel milk. Today, nut milks are also gaining popularity.
Most likely central Asia, when nomadic people domesticated cattle for milk.
Dairy farming spread with the domestication of mammals throughout Asia, Europe, and Africa.
For most of history, stored milk was dangerous to consume because it spoiled so quickly. It was not until the invention of pasteurization in the 1860s that farmed cattle milk became a popular beverage.
“Milk” from Encyclopedia Britannica https://www.britannica.com/topic/milk
The Oxford Companion to Food. 3rd ed., by Alan Davidson and edited by Tom Jaine (2014)
“The Surprisingly Intolerant History of Milk” by Daniel Fernandez (2018) https://www.smithsonianmag.com/history/ surprisingly-intolerant-history-milk-180969056