mon ‧ o ‧ ma ‧ ni ‧ a /ˌmɒnəʊˈmeɪniə $ ˌmɑːnoʊ-/ BrE AmE noun [uncountable] formal
a very strong interest in one particular idea or subject which prevents you from thinking about anything else:
an obsession with food that verged on monomania
—monomaniac /-niæk/ noun [countable]