Mammalia

The highest class of living organisms; it includes all the vertebrate animals (monotremes, marsupials, and placentals) that suckle their young, possess hair, and (except for the egg-laying monotremes) bring forth living young rather than eggs. [L. mamma, breast]

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Mam·ma·lia mə-'mā-lē-ə n pl the highest class of the subphylum Vertebrata comprising humans and all other animals that nourish their young with milk secreted by mammary glands, that have the skin usu. more or less covered with hair, a mandible articulating directly with the squamosal, a chain of small ear bones, a brain with four optic lobes, a muscular diaphragm separating the heart and lungs from the abdominal cavity, only a left arch of the aorta, warm blood containing red blood cells without nuclei except in the fetus, and embryos developing both an amnion and an allantois, and that except in the monotremes reproduce viviparously

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Mam·ma·lia (mə-malґe-ə) a class of warm-blooded vertebrate animals, including all that possess hair and suckle their young. It includes three major groups: placentals and marsupials, which are viviparous, and monotremes, which are oviparous.