In expository writing, a topic sentence is a sentence that summarizes the main idea of a 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, in academic essays they often appear at the beginning. 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. As the topic sentence encapsulates the idea of the paragraph, serving as a sub-thesis, it remains general enough to cover the support given in the body paragraph while being more direct than the thesis of the paper.