subtopic

/ˈsʌbˌtɑpɪk/
noun
  1. A topic that is part of a larger, more general topic.
    • The main topic was climate change, and one subtopic was renewable energy.
    • In history class, the teacher divided the lesson into subtopics like the causes and effects of the war.
    • For your essay, choose a subtopic that interests you within the broader subject of ancient Rome.