monomania \mon-uh-MAY-nee-uh; -nyuh\, noun
1. Pathological obsession with a single subject or idea.
2. Excessive concentration of interest upon one particular subject or idea.
Fred has an obsession with rubber ducks. It is all he can ever think about. It is a monomania that approaches a frenzy in which girlfriends or wife, family and sleep, mean nothing.
Monomania is derived from the Greek elements mono-, "one, single, alone" + mania, "madness, frenzy, enthusiasm."