The topic sentence is the condensation of the idea that paragraph expresses. The topic sentence expresses the core of the idea. All other sentences revolve around and support the topic sentence.
The topic sentence usually appears at the start of the paragraph, but it may appear in the middle, end or anywhere else.
Here is an example of a paragraph with topic sentence “Racism is rife in the United States.”
Racism is rife in the United States. The two main groups, whites and blacks, and are in a constant power struggle. The white race brought the black race to the United States as slaves many centuries ago. The slaves were treated cruelly under harsh conditions. The civil war of United States was fought to free the slaves, and it cost the lives of hundreds of thousands of white Americans. Today, there are many whites and blacks living like brothers and sisters in the United States. But, inspite of this, a general contempt for each other prevails between the races. Will racial tensions in the United States ever become a “thing of the past”?
Rife means “common or generally existing.”