* Adjective: This is the most common use. It describes something that relates to or involves a fiduciary duty. For example: "The lawyer has a fiduciary responsibility to their clients."
* Noun: This is less common but still possible. It refers to a person who holds a position of trust and responsibility for another person's assets. For example: "The executor of the will was appointed as the fiduciary of the estate."
Therefore, the specific part of speech depends on its context within a sentence.