"monomania"
Jean-Étienne Dominique Esquirol
Public DomainJean-Étienne Dominique Esquirol - Credit: Painted by Ch. Bazin, lithography by de Delpech,1838

In 19th century psychiatry, monomania was a form of partial insanity, conceived as single pathological preoccupation in an otherwise sound mind.  The term was coined by the French psychiatrist Jean-Etienne Dominique Esquirol around 1810.  The idea was that, rather than being totally insane, a particular part of the sufferer’s psyche was diseased and deranged while in other facets he was normal.

Esquirol differentiated between emotional monomania, where the patient is obsessed with only one emotion, and intellectual monomania, where the patient has only one kind of delirious idea.

Monomania was retained as one of seven recognized categories of mental illness in the 1880 US census. However, its importance as a diagnostic category declined from the mid-19th century.