Noun

1. excessive interest or concentration on a singular object or subject

   It was apparent to all but himself that what was once idle curiosity had become a monomania.

2. a pathological obsession with one person, thing or idea

Quotations

• 1905 — Arthur Conan Doyle, The Adventure of the Six Napoleons

  "There are no limits to the possibilities of monomania," I answered. "There is the condition which the modern French psychologists have called the 'idee fixe,' which may be trifling in character, and accompanied by complete sanity in every other way. A man who had read deeply about Napoleon, or who had possibly received some hereditary family injury through the great war, might conceivably form such an 'idee fixe' and under its influence be capable of any fantastic outrage."