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]

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "monomania":
   absorbed attention, absorption, application, complex, compulsion,
   concentration, contemplation, deep study, deep thought, engagement,
   engrossment, fascination, fixation, fixed idea, hang-up,
   hypercathexis, idee fixe, immersion, intentness, involvement,
   irresistible impulse, meditation, morbid drive, obsession,
   obsessive compulsion, possession, preoccupation, prepossession,
   profound thought, rapt attention, ruling passion,
   single-mindedness, studiousness, study, submersion, tic



Bouvier's Law Dictionary, Revised 6th Ed (1856):

MONOMANIA. med. jur. Insanity only upon a particular subject; and with a
single delusion of the mind.
     2. The most simple form of this disorder is that in which the patient
has imbibed some single notion, contrary to common sense and to his own
experience, and which seems, and no doubt really is, dependent on errors of
sensation. It is supposed the mind in other respects retains its
intellectual powers. In order to avoid any civil act done, or criminal
responsibility incurred, it must manifestly appear that the act in question
was the effect of monomania. Cyclop. Pract. Medicine, title Soundness and
Unsoundness of Mind; Dr. Ray on Insanity, Sec. 203; 13 Ves. 89; 3 Bro. C. C.
444; 1 Addams' R. 283; Hagg. R. 18; 2 Addams' R. 102; 2 Addams' R. 79, 94,
209; 5 Car. & P. 168; Dr. Burrows on Insanity, 484, 485. Vide Delusion;
Mania; and Trebuchet, Jur. de la Med. 55 to 58.