A search engine is provided that includes a refinement mechanism. The search engine receives and parses a search expression and delimits the expression into individual terms or words. The refinement mechanism includes a directory of related terms and a directory of compound terms. The refinement mechanism also includes a hierarchical data structure of refinement terms. The mechanism then identifies related terms for the individual terms in the search expression, compound terms that may be formed from individual terms in the expression, and terms in the expression that have sub structures or “sub trees” in the hierarchical data structure. An interface is generated in a graphical tree structure to present identified related terms, compound terms, and refinement terms. Based upon user interaction with the interfaces, the mechanism forms an enhanced search expression.