1. A computer-implemented method to associate a semantic cluster with one or more categories of a predefined taxonomy, the method comprising: a) accepting, by a computer system including at least one computer, a plurality of semantic clusters of re-occurring terms within a document, and having a frequency based on the reoccurrence of the term; b) identifying, by the computer system based on the accepted clusters of re-occurring terms within the document, one or more concepts for the document, each concept identifying different re-occurring terms having identical meanings; c) scoring, by the computer system, the identified one or more concepts, the score of each of the one or more concepts weighted by cluster frequency of each of the re-occurring terms identified by said concept; d) identifying, by the computer system, a set of one or more categories using at least some of the one or more scored concepts to look up one or more categories in a concept-category index, wherein a category corresponds to a node of the predefined taxonomy, which defines a structured set of categories; and e) associating, by the computer system, at least some of the one or more categories with the semantic cluster.