Which word best describes something that is never ending or changing?

The term that best describes something that is never ending or changing is "perpetual." This word emphasizes the notion of continuity without interruption, often implying an ongoing, unceasing cycle that does not vary over time. In contexts where something remains the same or unaltered indefinitely, "perpetual" captures the idea of an infinite duration or existence.

While "endless" and "ceaseless" also suggest a lack of termination, they may not necessarily imply the same enduring quality that "perpetual" conveys. "Unchanging," on the other hand, focuses specifically on the lack of change rather than the concept of time. Therefore, as a descriptor for something that is both never ending and unchanging, "perpetual" is the most fitting choice.