As has been stated earlier, SEI does not refer to the term Certification rather it refers to it as Assessment. So in context of CMMI, it is CMMI Assessment and not CMMI Certification.
In CMMI context when an organization goes for a CMMI Maturity Level, it is known as that the organization has achieved a particular CMMI Maturity Level for e.g. Organization XYS has achieved CMMI Maturity Level 3. And when an organization has gone for CMMI Assessment it is known as that the Organization XYZ has been assessed for CMMI Maturity Level 3 (or other level).
For more information please refer to a detailed post on CMMI Certification and its difference from CMMI Assessment.