I found this great word on dictionary.com. I think it should be used more often in the general population.
idee fixe \ee-day-FEEKS\, noun:
An idea that dominates the mind; a fixed idea; an obsession.
For example: Smelling the freshly baked brownies presented a huge temptation for me and became an idee fixe that could not be satiated until I had eaten the entire pan.