Real Dictionary

mammal in American English
any of a large class (Mammalia) of warmblooded, usually hairy vertebrates whose offspring are fed with milk secreted by the female mammary glands
noun: any vertebrate of the class Mammalia, having the body more or less covered with hair, nourishing the young with milk from the mammary glands, and, with the exception of the egg-laying monotremes, giving birth to live young

mammal in British English
noun: any animal of the Mammalia, a large class of warm-blooded vertebrates having mammary glands in the female, a thoracic diaphragm, and a four-chambered heart. The class includes the whales, carnivores, rodents, bats, primates, etc