The best replacement for topic is theme.

The best alternatives to topic are rated as following:
★★★★★  theme
★★★★★  subject
★★★★  main idea
★★★★  thesis
★★★  queue
★★  issue
★★  thesis statement
★  title
★  queue jms
★  concept
★  scope
  focus
  subject japanese
  subject matter
  category
  off-topic
  topic
  queue message
  thesis sentence
  spencer gifts
  topic sentence
  hollister

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Speaking about topic, one probably shouldn't forget about thesis statement.

Other search terms related to topic and their ratings are:
★★★★★ thesis statement
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topic
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