Which term refers to something that lasts for an indefinitely long time?

The term that refers to something that lasts for an indefinitely long time is "perennial." This word is often used to describe plants that grow year after year without needing to be replanted, symbolizing endurance and persistence over time. In a broader context, perennial can apply to concepts, ideas, or themes that recur continuously or consistently.

In contrast, the other terms do not capture the essence of lasting indefinitely. "Sporadic" refers to something occurring at irregular intervals, not continuously. "Imperturbable" describes a state of being calm and not easily upset or excited; it does not imply longevity. "Superfluous" means unnecessary or excess, which also does not relate to duration. Therefore, "perennial" is precisely the term that encapsulates the idea of something enduring for a long time.