What is a Mammal? Definition of MAMMAL 1 : any of a class (Mammalia) of warm-blooded higher vertebrates (as placentals, marsupials, or monotremes) that nourish their young with milk secreted by mammary glands, have the skin usually more or less covered with hair, and include humans.
African Serval Capybara
Mammals are warm-blooded; meaning they can regulate their own body temperature, give birth to live young, produce milk for their young, and to some extent, have or had hair.
Mammals can be found on all seven continents, and in every environment; desert, frozen tundra, forest, water, caves, underground, mountains, cliff sides, etc.