Content Management Interoperability Services (CMIS) is an open standard that allows different content management systems to inter-operate over the Internet. Specifically, CMIS defines an abstraction layer for controlling diverse document management systems and repositories using web protocols. CMIS defines a domain model plus web services and Restful AtomPub (RFC5023) bindings that can be used by applications.

For a detailed discussion of CMIS see: 

Content Management Interoperability Services (CMIS)