(pronounced "shock") Short for Semantically Interlinked Online Communities SIOC is a project that proposes a method for interconnecting blogs, forums and mailing lists to each other on the Semantic Web. SIOC started with the development of the SIOC core ontology, which is used to describe the domain of online communities and what they consist of users and posts and descriptions of other simple terms that occur in online communities. This structure that is created in online communities is often hidden in some database behind the scenes, and SIOC is used to expose that structure using semantics.
See "SIOC-ing the Semantic Web" on SemanticWeb.com.
See also "What Is The Semantic Web?" in the "Did You Know..." section of Webopedia.