The term search engines use for the formulae they use to determine the rankings of your Natural Listings. Search engines will periodically send a Spider through your website to view all its information. Their programs analyze this and other data to value your site and determine your relevance to a particular search as well as your ranking within that search. These algorithms can be very complicated (Google alone currently uses 106 different variables) and search engines closely guard their algorithms as trade secrets. Having a general understanding of the principles of the algorithm as they update is vital to SEO, as websites that Google holds in higher regard are more likely to appear higher within the rankings.