As near as I understand it, paranoia is a diagnosed condition whereby the person suffers from delusional thoughts which make the person hostile, suspicious, and sometimes aggressive. Monomania is an older term and not a diagnosed condition today. It refers to intensely obsessive thoughts in an otherwise normal mind. Paranoia is a well used term and most people will understand it in conversation. Whereas monomania is almost totally unused and most people will not understand it's use. Hope that helps :)