Interoperability is commonly referred as a software quality sub-characteristic. Some proposals for formalizing interoperability include sets of concepts linked to this phenomenon. Such concepts differ among the proposals, resulting in a different terminology for referring to the same thing. Non-unified terminology of interoperability concepts exhibits three main problems: homonymy, synonymy and missing concepts. Due to such problems, crucial aspects of the interoperability process could be either misunderstood or just omitted, resulting in an ambiguous and incomplete description of interoperability. To solve these problems, we propose a terminology unification of interoperability concepts. This unification is based on published studies about the interoperability formalization. Results lead us to recognize six main concepts useful to unify the interoperability vocabulary.