(covers information from several alternate timelines)
The Capitoline Wolf breastfeeding Romulus and Remus
Breast feeding or nursing was the act of using one's breast to feed milk to a newborn infant. In Humans, females are geared for nursing the young. (TNG: "The Outcast") One who breastfed a child that was not their own was called a wet nurse. (VOY: "The Q and the Grey")
In 2361, during the Bajoran Occupation, Kira Nerys was conducting reconnaissance in the Bestri Woods when she thought she saw a Cardassian and opened fire. After hitting the target she discovered that she had killed a mother hara cat that was nursing her young. (DS9: "Second Skin")
When William T. Riker was hesitant to try gagh in 2365, Klag jokingly suggested that he might as well have one of the Klingon females breast feed him, thereby insulting him by comparing him to an infant. (TNG: "A Matter Of Honor")
In 2367, Junior's feeding off energy from the USS Enterprise-D was compared to a baby nursing from its mother. (TNG: "Galaxy's Child")
In 2371, during Jadzia Dax's zhian'tara, Audrid Dax, in the body of Quark, spoke of the joy of holding a baby to one's breast and nursing it. (DS9: "Facets")
In 2372, shortly after giving birth on a duplicate of the USS Voyager, Samantha Wildman planned on breast feeding her newborn daughter, Naomi. The Doctor informed her that, since she lacked the scales of Ktarians, she would be unable to breast-feed in three to four weeks when Naomi's incisors erupted. (VOY: "Deadlock")