* Noun: When referring to the fundamental concept of energy in physics or the capacity to do work, "energy" is a noun.
* Example: "The sun is a source of energy."
* Adjective: When describing something as full of energy or vitality, "energy" can function as an adjective.
* Example: "She gave an energy-filled performance."
* Verb (rare): In very specific contexts, "energy" can be a verb, meaning to give energy to or energize.
* Example: "The music energied the crowd."
So, the specific part of speech for "energy" depends on the context of the sentence.