Al-Mubaid et al. proposed a semantic similarity measure in their research paper . They see ontologies as connected graphs but refer to clusters within ontology graphs without ever defining what they are.
I am not sure what they meant with clusters.
There are two possible ways to interpret the definition of clusters:
- The clusters are separate ontologies.
- every root node's branch forms an independent cluster?
 Al-Mubaid H, Nguyen HA. A cluster-based approach for semantic similarity in the biomedical domain. Conf Proc IEEE Eng Med Biol Soc. 2006;2006:2713-2717. doi:10.1109/IEMBS.2006.259235