Originally, it was an adjective, but it’s usually used as a noun meaning a newspaper, magazine, or professional journal. The adjective from the same Latin root is generally “periodic” in modern English. Note that most adjectives cannot be used as a noun. In your example, we know it’s a noun in context, since the article “the” is used, there is no noun following “periodical,” and “the periodical” is the subject of the verb “includes.” If “periodical” were an adjective, it would be followed by a noun, which is not the case in your example.

In the example the word periodical is a noun. A periodical is a newspaper or magazine. The word periodically is an adverb. "I take my car to the mechanic periodically to make sure everything is in proper working order."

Periodical is a noun. It means something similar to "newspaper."