The Collaborative International Dictionary of English v.0.48:

Monomania \Mon`o*ma"ni*a\, n. [Mono- + mania.]
   Derangement of the mind in regard of a single subject only;
   also, such a concentration of interest upon one particular
   subject or train of ideas to show mental derangement.
   [1913 Webster]

   Syn: Insanity; madness; alienation; aberration; derangement;
        mania. See Insanity.
        [1913 Webster]

WordNet (r) 3.0 (2006):

monomania
    n 1: a mania restricted to one thing or idea [syn: monomania,
         possession]