The defining characteristic of mammals is the presence of mammary glands, which produce milk to nourish their young. This feature is critical for the survival of mammalian offspring, as it allows for direct maternal care. In addition to mammary glands, mammals are warm-blooded (endothermic), have hair or fur on their bodies, and possess a unique structure in their middle ear that aids in hearing. These characteristics distinguish mammals from other animal groups and are crucial for their adaptability and evolutionary success in diverse environments.