ectotherm

/ˈɛktoʊˌθɜrm/
noun
  1. An animal that relies on external sources, such as the sun, to regulate its body temperature; a cold-blooded animal.
    • As an ectotherm, a snake's activity level depends on the temperature of its surroundings.
    • Lizards are classic ectotherms that bask on rocks to warm up.
    • Many fish are ectotherms, so their body temperature changes with the water around them.