There have been limited efforts so far to provide a detailed conceptualization and formal ontology for e-Participation. Current e-Participation literature is replete with fragmented models, which only partially describe aspects of e-Participation. Consequently consistent descriptions and comparison of e-Participation initiatives is difficult. In addition, no e-Participation ontology exists, which supports citizen-led e-participation on the Social Media platforms. This work bridges this gap by providing a detailed conceptualization and corresponding formal and executable ontology for e-Participation. These semantic models cover the core perspectives of e-Participation; democratic process, initiative and sociotechnical system. The developed models also explicitly support the integrated citizen- and government-led model of e-Participation. Results from the use of the ontology in describing two e-Participation initiatives at Local Government or County and European levels are also presented.