By the grammar of a language is meant either the relations born by the words of a sentence and by sentences themselves one to another, or the systematized exposition of these.
— Topic sentence of the Grammar article, Encyclopædia Britannica 1911 Edition
In prescriptive grammar, the topic sentence is 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, 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.