Ontologies are more and more adopted to provide knowledge sharing and reuse, and to promote cooperation between several scientific fields. An increasing need of cooperation and resource sharing is emerging in the electromagnetic arena as well. The range of potential semantic-based applications seems to be wide and the knowledge to be codified is complex and diverse. Therefore the implementation of a well structured ontology in the electromagnetic domain is important. This paper describes a proposal for an electromagnetic ontology framework. The main design criteria were reusability and shareability, in order to make electromagnetic knowledge embeddable in larger and more general frameworks. The implementation of an ontology for aperture-antenna array design is provided as a specific example of a practical application.