The name given to search results that are not sponsored (also often called organic results).
These are algorithmically determined by the search engine in question, with the
ranking and inclusion of search results based upon the quality of the sites being ranked, and the relevance to the keyword being used. Sponsored results (such as Google AdWords adverts) are often displayed next to the natural search results on the same page, depending on the search engine.
Although natural search results are often perceived as being more “honest” than paid-for results, they are still open to manipulation and spamming by black hat operators (although the search engines invest a lot of time and money in reducing this problem).