What adjective describes something not subject to change?

The adjective that describes something not subject to change is "immutable." This term implies a state of being unchangeable or unalterable, often referring to laws, principles, or characteristics that remain consistent over time. In various contexts, such as science or philosophy, the concept of immutability plays a crucial role, particularly when discussing fundamental truths or universal laws that do not vary regardless of circumstances.

The other choices refer to different qualities. "Impermanent" suggests a temporary state, indicating that something is subject to change or destined to last only for a limited time. "Impervious" describes an inability to be affected by something, such as water or criticism, but does not convey a lack of potential for change. "Insipid" describes something that is bland or lacking in flavor, which is unrelated to the concept of changeability or stability. Thus, "immutable" stands out as the most accurate choice for describing something that remains constant and unchanging.