A Formal ontology is an ontology with a structure that is guided and defined through axioms. The goal of a formal ontology is to provide an unbiased (domain- and application-independent) view on reality. Formal ontologies are founded upon a specific Formal Upper Level Ontology, which provides consistency checks for the entire ontology and, if applied properly, allows the modeler to avoid possibly erroneous ontological assumptions encountered in modeling large-scale ontologies.

[http://en.wikipedia.org/wiki/Formal_ontology]