A psychiatric term for an excessive, obsessive preoccupation with a single idea or subject. In everyday Japanese, it can also refer to an extreme fixation or obsession, similar to 'monomania' in English.
His monomaniacal collecting habit is astonishing.
That writer has a monomaniacal fixation on a particular theme.
Borrowed from English 'monomania', ultimately from Greek 'monos' (single) and 'mania' (madness).