Many access control models (ACM) have been proposed in the literature pertaining to specific requirements of different organisations and scenarios. Few approaches were made to provide better understandability of different aspects of ACM, like constraints, with the help of ontology. A reference ontology is needed to provide common semantics to understand and compare different ACMs of different systems and organisations which collaborate to work together like in Computer Supported Collaborative Work (CSCW). In this paper, we identify the common concepts across different ACMs and propose a generic ontology based on these concepts. Specific ontologies of different ACMs can be derived from this reference ontology. We demonstrate its applicability in some examples and make suggestions for its adaptability to suit more generic applications and scenarios.