i apologize for posting a web site that contains advertisements that may slant the information provided. sadly, this one does too, it's the way of the internet i guess, but it's main focus is the definitive definition and criteria for major depression according to the DSM IV (the Diagnostic and Statistical Manual of Mental Disorders).

http://depression.about.com/cs/diagnosis/a/mdd.htm

there is a link at the bottom for an article about "is it depression or sadness?" hopefully this may be more helpful.