Which word describes a condition that is never-ending or eternal?

The term that best describes a condition that is never-ending or eternal is "incessant." This word is often used to refer to something that continues without interruption, conveying the sense of an unceasing occurrence or relentless continuity. When applied in various contexts, "incessant" typically enhances the idea of ongoing action or presence that lacks a definitive end or pause.

In contrast, the other terms do not encapsulate the idea of being never-ending effectively. "Impermeable" refers to the quality of not allowing fluid to pass through, which is unrelated to the concept of time or continuity. "Inane" describes something that is silly or lacking sense and does not pertain to duration or continuity. Similarly, "incumbent" relates to someone who holds a particular office or position, without any implications of an ongoing condition or eternal aspect. Thus, "incessant" stands out as the most fitting word for describing an unending condition.