It is currently in its third revision (ICD-O-3). ICD-10 includes a list of morphology codes. They stem from ICD-O second edition (ICD-O-2) that was valid at the time of publication.
The morphology axis is for the morphology of the tumor.
This axis has additional importance, because the Systematized Nomenclature of Medicine(SNOMED) has adopted ICD-O's classification of morphology. SNOMED has been changing continuously, and several different versions of SNOMED are in use. Accordingly, mapping of ICD-O codes to SNOMED requires careful assessment whether entities really match, or not.