Hit to fall under Natural Search, following conditions are checked.
1. Referrer matches the list of Adobe Identified Search engines.
2. Keyword query markup is present in referrer URL. (It ensures that hit from a non search engine part of a search engine domain is not considered as a search. For example mail.google.com, it is not a search engine though it is part of google.com). Keyword query markup for Google search engine is 'q='.
3. Query string parameter in the landing Page URL is not present or doesn't match Paid search Detection rules (Paid Search Detection).
If all of the above are true for a hit then it is considered as a Natural Search.
A hit from a search engine that is not Paid is considered as Natural Search. ( Natural Search = Search - Paid Search)