What does heterothermic mean?
heterothermic
1. of animals except birds and mammals; having body temperature that varies with the environment
From Wikipedia
Heterothermic (from Greek: hetero = “other” thermy = “heat.”) is a physiological term referring to a unique case of poikilothermy. Heterothermic creatures are homeothermic for a portion of the day, or year. More often than not, it is usually used as a way to dissociate the fluctuating metabolic rates seen in some small mammals and birds (e.g. bats and hummingbirds), from those of traditional cold blooded animals. In many bat species, body temperature and metabolic rate, are elevated only during activity. When at rest, these animals reduce their metabolisms drastically, which results in their body temperature dropping to that of the surrounding environment. This makes them homeothermic when active, and poikilothermic when at rest.
Note: Strictly speaking, heterothermy is just a variant of poikilothermy, as the internal body temperature still varies.