What is the term for a lack of energy and enthusiasm?

The term that describes a lack of energy and enthusiasm is "lethargy." It specifically refers to a state of sluggishness, inactivity, or lack of interest, which encompasses the feeling of being unable to muster energy for various activities. This term captures not just physical tiredness but also a broader disengagement or indifference that can accompany a low energy state.

While "fatigue" and "exhaustion" relate to physical or mental tiredness, they do not explicitly convey a sense of lack of enthusiasm. "Apathy," on the other hand, refers more to a lack of interest or concern more broadly, not necessarily tied to the physical aspect of energy levels. In contrast, "lethargy" effectively combines both the diminished energy and enthusiasm components, making it the most fitting choice in this context.