Defining Characteristics:

* Mammary glands: Mammals are the only animals that possess mammary glands, which produce milk to nourish their young. This is the most fundamental characteristic that gives them their name.

* Hair or fur: Mammals have hair or fur at some point in their lives, even if it's only present as a fetus or a newborn. This provides insulation, helps with camouflage, and aids in sensory functions.

* Three middle ear bones: Mammals have three bones in their middle ear (malleus, incus, and stapes) that transmit sound vibrations from the eardrum to the inner ear. This allows for a more sophisticated sense of hearing.

* Warm-blooded: Mammals are endothermic, meaning they can regulate their own body temperature. This allows them to be active in a wider range of environments compared to cold-blooded animals.

* Four-chambered heart: Mammals have a four-chambered heart that separates oxygenated and deoxygenated blood, providing a more efficient circulatory system.

* Diaphragm: Mammals have a diaphragm, a muscular sheet that separates the chest cavity from the abdominal cavity. This helps with breathing.

* Live birth: Most mammals give birth to live young, with only a few exceptions (like the platypus and echidna).

* Neocortex: Mammals have a neocortex, a part of the brain associated with higher cognitive functions like learning, memory, and sensory perception.

Other Common Features:

* Teeth: Most mammals have specialized teeth for different functions, such as biting, grinding, and tearing.

* Sweat glands: Many mammals have sweat glands to help regulate body temperature.

* Complex social behavior: Many mammals exhibit complex social interactions and behaviors, including cooperation, communication, and territoriality.

Important Note: While these characteristics are generally true for mammals, there are exceptions. For example, some whales have lost their fur, and some monotremes (like the platypus) lay eggs instead of giving birth to live young. However, these exceptions are rare and do not change the overall definition of a mammal.