Final answer:
The term most synonymous with ectotherm is poikilotherm, as both terms describe organisms that have varying internal temperatures dependent on the environment, unlike endotherms which maintain a constant body temperature.
Step-by-step explanation:
The term most synonymous with ectotherm is poikilotherm. Ectotherms are organisms that rely primarily on external heat sources to maintain their body temperature. They do not generate sufficient metabolic heat to maintain a constant internal body temperature but instead, exhibit body temperatures that vary with the environment. This characteristic is similar to that of poikilotherms, whose internal temperature varies considerably. In contrast, an endotherm is capable of maintaining a constant body temperature regardless of environmental temperatures, thanks to their ability to produce internal metabolic heat.