Multimedia Content is described via textual, semantic and structural Descriptors and Description Schemes, as introduced in MPEG-7. The semantic part of the description is closer to what the user expects from a multimedia search engine, however it poses difficulties, because of the potential incompatibility of the semantic entities among different archives. In this paper, we present FAETHON, a system that unifies the semantic description of heterogeneous archives, through the use of a semantic encyclopaedia.