Listings that appear on a search engine solely because of merit, applicability, etc. In other words, listings that are not paid for; also called natural listings.
Organic listings are the results from a Search Engine where the web pages which appear have not been paid for but appear only because they are deemed important to the search which has been made. The alternatives to organic listings are Paid Inclusion and Pay Per Click listings.
On many search engines with in the results pages there will be sites that have paid to be there as well as the ons that have got their by their own merits, the free listings are also known as the organic listings.