The topic sentence is a primarily prescriptive grammatical

Grammar

In linguistics, grammar is the set of structural rules that govern the composition of clauses, phrases, and words in any given natural language. The term refers also to the study of such rules, and this field includes morphology, syntax, and phonology, often complemented by phonetics, semantics,...

 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

Summary

A summary, synopsis, or recap is a shorter version of the original. Such a simplification highlights the major points from the much longer subject, such as a text, speech, film, or event...

, 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

Thesis statement

The thesis statement states the thesis or argument of the author in an essay or similar document. Usually no less than five sentences or six long, it is a focused section of text that clearly delineates the argument that is presented in the work and is usually found at the end of the first...

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

Its use is considered standard in the American educational system and most venues of writing mainly because it increases reading accessibility.

A topic sentence (also known as a focus sentence) encapsulates or organizes an entire paragraph, and a writer should be careful to include one in most major paragraphs. Although topic sentences may appear anywhere in a paragraph, in academic essays they often appear at the beginning.

The sentence in bold in the first paragraph is an example of a topic sentence.