Here's why:
* It describes a noun: It tells us something about a person, place, or thing. For example: "The sunset was glorious."
* It can be used as a noun: But it's less common. You might say, "The glorious of the mountains is breathtaking." Here, "glorious" refers to the magnificence of the mountains.
So, it's most often an adjective, but it can act as a noun in certain contexts.