A topic sentence is the most important sentence in a paragraph. Sometimes referred to as a focus sentence, the topic sentence helps organize the paragraph by summarizing the information in the paragraph. In formal writing, the topic sentence is usually the first sentence in a paragraph (although it doesn't always have to be).
Read more at http://examples.yourdictionary.com/examples-of-topic-sentences.html#z0FPmJOwYLAGXXtD.99