Topic sentence

 The topic sentence is a primarily prescriptive grammatical term to describe the sentence in an expository paragraph which summarizes the main idea of that paragraph

 

  It is usually, but not always, the first sentence in a paragraph. The topic sentence acts as a kind of summary, and offers the reader an insightful view of the writer’s main ideas for the following paragraph

 More than just being a mere summary, however, a topic sentence often provides a claim or an insight directly or indirectly related to the thesis

  It adds cohesion to a paper and helps organize ideas both within the paragraph and the whole body of work at large.

 

A topic sentence (also known as a focus sentence) encapsulates or organizes an entire paragraph. Although topic sentences may appear anywhere in a paragraph, in academic essays they often appear at the beginning.

more info :  http://en.wikipedia.org/wiki/Topic_sentence