1. Class Mammalia, a taxonomic class within the subphylum Vertebrata — the mammals. (wiktionary.org)
2. 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 usually 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. (merriam-webster.com)
3. Clade of warm-blooded amniotes. Among the features that distinguish them from the other amniotes, the reptiles and the birds, are hair, three middle ear bones, mammary glands in females, and a neocortex (a region of the brain). (wikipedia.com)
Word origin: From Modern Latin (Linnaeus), from neuter plural of Late Latin mammalis, from mamma.