"insanity in regard to a single subject or class of subjects; mental action perverted to a specific delusion or an impulse to do a particular thing," 1820, probably on model of earlier French monomanie (1819), from Modern Latin monomania, from Greek monos "single, alone" (from PIE root *men- (4) "small, isolated") + mania (see mania).
Men of one idea, like a hen with one chicken, and that a duckling. [Thoreau, "Walden"]
"morbid fear of everything," attested by 1848 in medical journals for a psychological condition described as "monomania, with fear and terror," from Greek pantos, neuter genitive of pas "all" (see pan-) + -phobia. Earlier (by 1819) it was noted as an old word for "hydrophobia, rabies." Related: Pantophobe; pantophobic.
The propensity which leads an insane person to accomplish his purpose by burning, has been considered to merit particular notice, and to constitute a variety of monomania. Dr. Marc, of France, has published a memoir on the subject; he gives the name of pyromania to it, and considers that, like other insane propensities, it may be the result of instinct, or it may be the result of delusion—reasoning upon erroneous principles. [Alexander Morrison, M.D., "The Physiognomy of Mental Diseases," London, 1840]
An older word for it was incendiarism.