We describe ONIONS, a methodology for integrating ontologically-heterogeneous taxonomic knowledge and its current application to medical domain. Some clarification is given of our intended meaning of ontology and related notions, then main problems of ontology design are addressed, with a short comparison with alternative approaches. The methodology is described as a sequence of phases. The top-level of the current integrated ontology of heterogeneous medical taxonomies is presented in an order-sorted logic. ONIONS includes no claim of global objectivity (it performs an integration of explicit ---or explicited--- ontologies of given taxonomic sources), but provides a feasible solution to the problems of modelling stopover and cognitive basicality. ONIONS has been defined in order to be applied to sources within the same domain, nevertheless it has been applied to a very wide and inherently heterogeneous domain like medicine, so complex that it can be considered in itself an integration of subdomains. 1.

ONIONS helps terminological ontology construction from existing, contextually heterogeneous terminologies. It is a methodology for integrating the context-dependent conceptualizations underlying conceptually heterogeneous terminology systems. We describe an application of this methodology to the medical domain with an example extracted from the UMLS system. We also give a short description of the current ontology library produced by means of ONIONS, and of its metaontology.