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 illness
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