The Semantic Desktop needs a well-thought use of ontologies and ontology languages. Existing ontology languages like RDF/S, OWL, SKOS and Topic Maps are very well suited for certain application areas, but do not fulfill some of the requirements given on the Semantic Desktop. In this report, a new ontology language, extending RDF/S, is proposed, the PIMO ontology language, which addresses the requirements of the Semantic Desktop and uses existing solutions as an inspiration to build a suitable solution. The language contains a core upper ontology, defining basic classes for things, concepts, resources, persons, etc. and also stops at these basic entities. Extending the ontology definitions of classes and relations is possible by PIMO-domain ontologies. The core application area of the PIMO-language is to allow individual persons to express their own mental models in a structured way, the different mental models can then be integrated based on matching algorithms or on domain ontologies. Based on the core upper ontology elements, each user can extend his personal mental model in an open manner. Accompanying to the description of the ontology is a RDF/S version of the ontology language, created using the popular Protégé tool. An example of the mental model of a user is given, the fictional user "Paul" is further described. With the gnowsis-beta open source software, an implementation based on the PIMO language exists, that allows validating ontology files. A web-service for that will be provided soon. The ontology language, this document, the open-source reference implementation and the example documents can provide a stable basis for discussions on this topic and allow you to extend your own work.