There is no agreed upon definition of fake news.

Some define it very narrowly: It is generally false or misleading information presented as news.

image credit: Huffington Post

How to Spot Websites that are Fake or are Mimicking:

• Look for contact information with a verifiable address and affiliation.
• Look for an About page, often in the header or footer of the home page. Read the About page closely for evidence of partisanship or bias.  If there's no About page and no Contact page, be very skeptical.
• In staff listings (or on the About page), look critically at the list of executives. Are they real people or stock photos? Open a new tab and look for another profile of the individual (e.g. LinkedIn).
• Perform an independent search for the news source. Compare and verify URLs.
  Example: http://abcnews.com.co/ (fake site) is not the ABC Network News, but the logo and the URL are almost identical.

Be on the lookout for satire (for example, The Onion).

Here is a Wikipedia page that contains all of the known fake websites: Wikipedia's list of fake news websites