What term refers to someone who is persistently stubborn or unyielding?

The term "obstinate" refers to someone who is persistently stubborn or unyielding, demonstrating a strong adherence to their own ideas or opinions despite attempts to persuade them otherwise. This characteristic often signifies a refusal to change one's mind or behavior even when there may be a better alternative or when faced with compelling arguments.

The choice highlights a quality of tenacity but in a negative light—suggesting an unwillingness to adapt or compromise. This aligns perfectly with the definition in the question.

In contrast, "defiant" implies a bold resistance to authority or control, which may not necessarily include the stubbornness implied by obstinacy. "Flexible" denotes a willingness to change and adapt, directly opposing the idea of being unyielding. Lastly, "submissive" describes a tendency to yield to others' wishes or authority, which is also contrary to the notion of being stubborn. Thus, "obstinate" is the most accurate term for someone who is persistently unyielding.