‘乳汁’ literally combines 乳 (rǔ), meaning 'milk' or 'breast', and 汁 (zhī), meaning 'juice' or 'liquid extract'. Together, they refer specifically to milk produced by mammals—especially human breast milk or milk from animals like cows, goats, or dogs. Unlike the more general term 牛奶 (niú nǎi, 'cow’s milk') or 母乳 (mǔ rǔ, 'mother’s milk'), 乳汁 emphasizes the biological, nourishing fluid secreted by mammary glands, often in scientific, medical, or zoological contexts.

This word carries a neutral-to-formal register and is rarely used in casual daily speech—people usually say 奶 (nǎi) or specify the source (e.g., 羊奶 yáng nǎi). It appears frequently in biology textbooks, pediatric health discussions, veterinary care, and breastfeeding guidance. While it can technically refer to any mammal’s milk, it most commonly evokes human lactation or animal nursing behavior in academic or caregiving settings.