Why are fish, amphibians, and reptiles called cold-blooded?

Amphibians, reptiles, fish are highly dependent on the ambient temperature, their body temperature becomes the same as the outside temperature, hence the name. Unlike warm-blooded animals, these species of animals become slower when cold weather sets in and go into hibernation. Warm-blooded animals are not affected by the ambient temperature, their body temperature always remains constant, so warm-blooded animals can continue their activity even when severe cold weather sets in.