Monomania

In psychiatry, monomania (from Greek monos, one, and mania, mania) is a type of paranoia in which the patient has only one idea or type of ideas. Emotional monomania is that in which the patient is obsessed with only one emotion or several related to it; intellectual monomania is that related to an only kind of delirious idea or ideas. See also mania, psychosis.

In colloquial terms, monomania is often attached to subcultures that to the general public appear esoteric. Hence terms like fanboy, anorak, otaku and so forth. The difference between monomania and passion can be very subtle and difficult to recognize.