ob·ses·sion 
(əb-sĕshən, ŏb-)
(əb-sĕshən, ŏb-)n.
1. A pattern of unwanted or intrusive thoughts or urges that recur persistently, often accompanied by symptoms of anxiety.
2. A compulsive, often unreasonable idea or emotion.
ob·session·al adj.
ob·session·al·ly adv.