An irrational, or abnormal fixation or preoccupation; an obsession.
Sexual attraction to or arousal at something abnormally sexual or nonsexual, such as an object or a nonsexual part of the body.
Something which is believed to possess, contain, or cause spiritual or magical powers; an amulet or a talisman.
That said to be possessed by someone who is obsessed with something or only able to think of one thing.