Apache Solr is a leading open source search platform built upon Lucene. Solr could be used as an enterprise search platform or for a consumer-facing website such as an eCommerce retail site. Taxonomy adds value in either case by providing structured metadata that Solr can leverage to provide advanced faceted or hierarchical navigation options so users can more quickly get to the right search result. Simply load in an entry, add the meta-columns to the document (either manually or by a “top n” search from Lucene that exceeds a certain threshold) coupled with the use of a hierarchy. You can then load the document that mentions the term "Denver, Colorado, USA." This allows you to store in the taxonomy as USA/Colorado/Denver and allows for filtering on whatever attributes you load.