Ontology alignment is one-to-one correspondence between specific entities of two ontologies. It might happen that for some entities no corresponding entity can be found.

Falconer, S.M., Noy, N.F., Storey, M.A.: Ontology mapping-a user survey. In: Proceedings of the Workshop on Ontology Matching (OM2007) at ISWC/ASWC2007, Busan, South Korea. pp. 113-125 (2007) [-]

Ontology alignment is the process of finding the similarities or relations between two different ontologies.

Klein, M.: Combining and relating ontologies: an analysis of problems and solutions. In: IJCAI-2001 Workshop on ontologies and information sharing. pp. 53-62. Citeseer (2001) [-]