Topic sentence

Topic sentence is a term in grammar. It applies to expository writing. Expository writing is a type of writing where the purpose is to explain, inform, or even describe. 
A topic sentence is usually the first in a paragraph. It states the main idea in a paragraph. In this way, it helps the reader to understand what follows. So, for example, a sentence like "Minor planets are smaller than major planets, and have orbits further out than the orbit of Neptune" could be followed by details of the minor planets. Often topic sentences do a similar job to summaries.