CMIS is a specification for improving interoperability between Enterprise Content Management systems. OASIS approved CMIS as an OASIS Specification on May 1, 2010. It proposes a data model plus a set of generic services and several HTTP-based protocol bindings for these services, including SOAP and REST/Atom.
The page describing how Nuxeo and CMIS work together is available here.
The official OASIS CMIS TC is here.