Here's what rhymes with externality.

Consider these alternatives

externalities / internalities cooperativity / activity undesired / desired inotropic / logic separability / stability hemagglutination / information mosaicism / given satisfiability / ability internalization / relation disequilibrium / equilibrium thioether / whether implication / education vignetting / getting amelioration / operation malocclusion / conclusion prognathism / given iatrogenic / academic disjunction / function