Semantic similarity, also called semantic relatedness or semantic closeness/proximity/nearness, is a concept whereby a set of documents or terms within term lists are assigned a metric based on the likeness of their meaning / semantic content.
Real life data needs are never semantically pure. Users need to browse their data in different ways. Hierarchies are too hard to reorganize on a whim. Stuff I need access to DOES NOT HAPPEN TO EQUAL the stuff at the top of the tree: Hierarchies are bad a