To lactate is to produce milk for feeding babies. All female mammals who give birth lactate.
A mother cow has to lactate in order to feed her calves, and so that she can be milked by a farmer too. Mothers who nurse their babies lactate, whether they're humans, cats, goats, or skunks. Lactation, "the secretion of milk," came first, from the Late Latin lactationem, "a suckling," from the root lac, "milk."