What term describes someone or something that is stubborn or unyielding?

The term that describes someone or something that is stubborn or unyielding is "adamant." This word conveys the idea of a firm resolution and an unwillingness to change one's mind or position, often in the face of persuasion or argument. Someone who is adamant stands firmly by their beliefs or decisions and is resistant to any attempts at influence or change.

In contrast, the other terms—the characteristics or states they describe—do not relate to unyielding behavior. For example, "docile" refers to someone who is easily managed or submissive and does not exhibit stubbornness. "Agreeable" denotes a willingness to accept or agree with others, suggesting flexibility rather than a fixed position. Lastly, "indecisive" describes someone who struggles to make decisions or is unable to commit, contrasting sharply with the firm decisiveness implied by being adamant. Thus, "adamant" is the definitive term for someone or something that is characterized by stubbornness and unyielding behavior.