Depression is a disorder of mood ...
[which] remains nearly incomprehensible to
those who have not experienced it in its
extreme mode, although ... the blues ...
give many individuals a hint of the illnes
in its catastrophic form.
-William Styron, Darkness Visible

Most people will experience episodes of normal depression resulting from common life circumstances such as physical illness, relationship difficulties, the loss of a job, the death of a friend.

However, depression can become an illness if -

• The mood state is severe.
• It lasts for 2 weeks or more.
• It interferes with our ability to function at home or at work.
  
  Links and resources: http://www.blackdoginstitute.org.au