Which word describes the quality of being unable to be subdued or overcome?

The term "indomitable" refers to a spirit or quality that cannot be subdued, conquered, or overcome. It suggests a sense of resilience and unwavering strength in the face of adversity. For instance, consider a person or movement that continues to strive and push forward despite challenges; they embody an indomitable spirit. This word captures the essence of being unbreakable and tenacious, making it the most fitting choice in the context of the question about being unable to be overcome.

While “invincible” implies that something cannot be defeated, which is close, it often refers more to physical prowess or success in battle as opposed to the underlying qualities of spirit and character that "indomitable" conveys. Similarly, “unyielding” and “inflexible” suggest a steadfastness or rigidity, but they do not encapsulate the broader sense of resilience and courage that is inherent in being indomitable. Thus, the choice of "indomitable" as the answer aligns perfectly with the quality described in the question.