What term describes the subject treated in a paragraph or work?

The term that most accurately describes the subject treated in a paragraph or work is "Topic." The topic serves as the specific focus of a piece of writing, pinpointing what the text is addressing or discussing. It is often a single word or phrase that encapsulates the essence of what the text is about.

While "Theme" refers to the underlying message or moral of a piece, and "Subject Matter" can denote broader or more complex concepts at play, "Topic" is the most precise term for what is specifically being discussed. "Idea," on the other hand, can be more abstract and does not necessarily pinpoint the central focus of a work in the same way that "Topic" does. Therefore, in this context, "Topic" is the clear choice for identifying the subject of a paragraph or work.