1) Part of an essay paragraph 
The topic sentence is a prescriptive grammatical term to describe the sentence in an expository paragraph which summarizes the main idea of that paragraph. It is usually the first sentence in a paragraph. Also known as a focus sentence, it encapsulates or organizes an entire paragraph. Although topic sentences may appear anywhere in a paragraph, i...
[n] - a sentence that states the topic of its paragraph
The sentence at the beginning of a paragraph, that includes the main idea of the paragraph.
noun a sentence that states the topic of its paragraph