Here's why:
* It describes a noun: "The defiant child refused to apologize."
* It modifies other words: "She gave him a defiant stare."
However, "defiant" can also function as a noun in some contexts:
* It can refer to a person who is defiant: "The defiant was eventually arrested."
* It can refer to the act of defiance: "They made a defiant stand against the injustice."
So, the part of speech depends on the context of the sentence.