Alligators are neither mammals nor amphibians; they are reptiles. More specifically, they belong to the order Crocodylia, which also includes crocodiles and caimans.
To clarify, mammals are warm-blooded vertebrates that typically have hair or fur and produce milk to feed their young. Amphibians, on the other hand, are cold-blooded creatures that usually undergo a metamorphosis from a juvenile aquatic form to an adult terrestrial form. Alligators do not fit into either of these categories.
Reptiles, like alligators, are characterized by their scaly skin, the laying of eggs on land, and their reliance on external sources of heat to regulate their body temperature. So, while alligators share some similarities with mammals and amphibians, they are distinct as reptiles.