For a woman or a female animal, to produce milk from the body to feed her baby. It is the medical or scientific word for it — doctors, nurses, midwives, and biology books use it; ordinary people say 'produce milk', or talk about breastfeeding. The two are not quite the same thing: lactating is the body MAKING the milk, breastfeeding is the baby DRINKING it, so a mother can lactate without breastfeeding (she may pump the milk instead). Because it is clinical, it sounds neutral and slightly distant, which is exactly why the medical world likes it. It also lives in a small family of related words you will meet on food packaging and in hospitals: lactation, lactose, and so on. Note that the same spelling is also a chemistry noun — lactate, a substance your muscles produce during hard exercise — but that is a different word with a different stress pattern, and not this one.