Taxonomy: In language, a taxonomy refers to a classification system, usually hierarchical, that is used to organize topical subjects. The term is also related to search engines and has to do with methods used in specialty -- or vertical -- search engines.
Many people hear the word "taxonomy" and think of the biological classification of organisms (Domain, Kingdom, Phylum, Class, Order, Family, Genus, and Species). This model serves as a good representation for how taxonomy for search engines works, too. When a keyword search is performed on a search engine or an individual website, results will show for that word and often for related words, as well. For instance, a Google search for "dogs" might also give information on "dog breeds" or "dogs for sale" due to the search engine's taxonomy.
Another good way to think of the taxonomy is to think of a tree. Even though a user might search for a term (on one branch), the search engine may also display relevant terms (from nearby branches).
This type of order is also found on vertical search engines, which are sometimes referred to as specialty search engines and are focused on a certain topic. An example of this is a site like Yelp, a site that helps users find restaurants, shops, and other businesses nearby. Because vertical search engines are only looking at a certain topic, they can use a more focused crawler to find relevant information. So if someone searches Yelp to find a vegetarian restaurant in New York City, they would see results for those types of businesses only. This is in contrast to using a general search engine, like Google, where the user may see useful results mixed in with not so useful ones (e.g. a steakhouse that has a single vegetarian option on the menu).