A hierarchical classification system. In the world of content, this can be a hierarchy of terms (generally called nodes or entities) that are used to classify the category or subject content belongs to as well as terms that are included in the content. In many cases, website navigation systems appear taxonomical in that users narrow down from broad top-level categories to the granular feature they want to see. An ontology is similar to a taxonomy in that it is also a classification system with nodes or entities, but it is more complex and flexible because ontologies allow for nonhierarchical relationships. While in a taxonomy a node can be either a broader term or narrower term, in an ontology nodes can be related in any way.