na-adjective, noun
A somewhat literary or technical term for an obsessive fixation on a single idea or subject. Can refer to the condition or a person with it.
彼は偏狂的なまでに切手収集に没頭している。
He is obsessed with stamp collecting to the point of monomania.
その発明家は偏狂と言われるほど一つの研究に打ち込んだ。
The inventor devoted himself to a single line of research to the extent that he was called a monomaniac.