Key nouns

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Across
  1. 2. Main subject of a talk, book etc.
  2. 4. Basic idea or rule that explains how something happens or works
  3. 6. A thing being discussed, considered or studied
Down
  1. 1. Main characteristic of something
  2. 2. Used to refer to what one is studying or writing about
  3. 3. A representation of something as a simple description and might be used in calculations
  4. 5. Subject or problem which people are talking about
  5. 7. Statement of the rules on which a subject of study is based/an opinion or explanation