This question came up in a group that I belong to. The answers varied and the definition couldn't really be agreed upon. However, I offer to you some ideas of what I believe creative non fiction is:
It uses fiction techniques such as characterization, plotting, setting and dialogue to write a piece of non fiction.
It uses actual people and events to tell a story.
It is written from personal experience or it tells the story of someone else.
It uses forms such as letter, memoirs, blog posts, biographies, autobiographies...
Wikipedia defines it as:
Creative nonfiction, also known as literary journalism and narrative journalism, which uses literary skills in the writing of nonfiction. A work of creative nonfiction, if well written, contains accurate and well-researched information and also holds the interest of the reader. Creative nonfiction is contrasted with "research nonfiction" which may contain accurate information, but may not be particularly well written and may not hold the attention of the reader very well.
If you are interested in joining The Writing Mother Creative Non Fiction group, head over here!