IPA: /ˌmɒnəˈmeɪniə/
KK: /mɒnəˈmeɪniə/
An intense and often unhealthy focus or obsession with a single idea or subject.
His monomania about collecting stamps made it difficult for him to enjoy other hobbies.
Monomania → It is formed from "mono-" (from Greek "monos", meaning single or alone) and "mania" (from Greek "mania", meaning madness or frenzy). The word describes a condition of being obsessed with a single idea or subject, reflecting a singular madness.
Think of being 'alone' ('mono-') in your 'madness' ('mania') — that's why monomania means an obsession with just one thing.