A lightweight ontology is an ontology representing a backbone taxonomy where the concept of the child node is more specific than the concept of the parent node. Formal lightweight ontologies can be generated from their informal ones. The key applications of formal lightweight ontologies are document classification, semantic search, and data integration. However, these applications suffer from the following problems: the disambiguation accuracy of the state of the art NLP tools used in generating formal lightweight ontologies from their informal ones; the lack of background knowledge needed for the formal lightweight ontologies; and the limitation of ontology reuse. This book describes a solution to these problems in formal lightweight ontologies; namely, faceted lightweight ontology (FLO). A detailed description -- of some experiments conducted in building an FLO on geographical domain, and of an XML like language called C-XML developed for representing FLO, is provided.