"The topic (which is introduced by the topic sentence) should be discussed fully and adequately. Again, this varies from paragraph to paragraph, depending on the author's purpose, but writers should beware of paragraphs that only have two or three sentences. It's a pretty good bet that the paragraph is not fully developed if it is that short."
In an essay the purpose of the development is two-fold: (1) to support the topic sentence and (2) to support the main idea of the thesis.
Source
Driscoll, Dana and Allen Brizee. “On Paragraphs.” 2010. OWL: Purdue Online Writing Lab. Purdue U.
15 Mar. 2010 <http://owl.english.purdue.edu/owl/resource/606/01/>.
What is "adequate development"?
A well-developed paragraph:
A topic sentence: Introduces the main idea of the paragraph (a mini-thesis).
Supporting sentences = development:
Typically, supporting sentences are of two types: general information and specific detail.
Ideally, each sentence connects to the previous sentence in some way (repetition of key terms or transitional words) then provides new information.
As a general rule, three supporting sentences are a good starting point. However, that is a minimum beginning point.
Think in terms of supporting the idea introduced by the topic sentence with evidence:
First, make a general statement that supports the topic sentence.
Next, for that general statement, provide specific detail: Help the reader "see" the idea.
Repeat as needed.
This support can be viewed as "evidence": For information about evidence - the types of information used for support/development and the "rules" of evidence, click here > Evidence
Additionally, support can come in the form of
commentaries
The expression of opinions or explanations about an event or situation,
interpretations
explaining the meaning of something, and
analyses
Detailed examination of the elements or structure of something, typically as a basis for discussion or interpretation
A concluding sentence: Returns to the main idea of the paragraph.
Sources
Dictionary.com for definitions: commentary, interpretation, analysis
Lorcher, Trent. “Teaching Paragraph Writing: Improve Paragraph Structure, and Know
Which Types of Paragraphs to Use.” Bright Hub. 12 Apr. 2010.
<http://www.brighthub.com/education/k-12/articles/22351.aspx>.