What is Unity in Writing?

From http://lrs.ed.uiuc.edu/students/fwalters/cohere.html "Unity is a very important characteristic of good paragraph writing. Paragraph unity means that one paragraph is about ONLY ONE main topic. That is, all the sentences -- the topic, supporting sentences, the detail sentences, and (sometimes) the concluding sentence -- are all telling the reader about ONE main topic. If your paragraph contains a sentence or some sentences that are NOT related to the main topic, then we say that the paragraph "lacks unity," or that the sentence is "off-topic." " In general, it means that your paper is coherent, easy to follow, and each paragraph relates to the main thesis. "

To help your paragraph unity try these things:
From: http://papyr.com/hypertextbooks/comp1/coherent.htm

"To achieve paragraph unity, a writer must ensure two things only. First, the paragraph must have a single generalization that serves as the focus of attention, that is, a topic sentence. Secondly, a writer must control the content of every other sentence in the paragraph's body such that (a) it contains more specific information than the topic sentence and (b) it maintains the same focus of attention as the topic sentence. "