"A reference architecture in the field of software architecture or enterprise architecture provides a template solution for an architecture for a particular domain. It also provides a common vocabulary with which to discuss implementations, often with the aim to stress commonality. " - Wikipedia.
"A reference model is an abstract framework for understanding significant relationships among the entities of some environment, and for the development of consistent standards or specifications supporting that environment. A reference model is based on a small number of unifying concepts and may be used as a basis for education and explaining standards to a non-specialist. A reference model is not directly tied to any standards, technologies or other concrete implementation details, but it does seek to provide a common semantics that can be used unambiguously across and between different implementations." - OASIS
The NIST Cloud Computing Reference Architecture is a detailed example of complete reference architecture. It includes a high level Reference Model along with details about the Actors, interactions and implementation patterns that are best practices for delivery and consumption of cloud services.