obsession /-seshˈən/ noun

1. A fixed idea
2. The morbid persistence of an idea in the mind, against one's will (psychiatry)
3. Persistent attack, esp of an evil spirit (archaic)
4. The state of being molested in this way from outside, opp to possession, or control by an evil spirit from within (archaic)
5. A siege (obsolete)

obsessˈional adjective

obsessˈionally adverb

obsessˈionist noun

Someone who is obsessed by a fixed idea

obsessˈive adjective

1. Relating to or resulting from obsession
2. Obsessing

Someone affected or characterized by obsessive behaviour

obsessˈively adverb

obsessˈiveness noun

obsessive-compulˈsive adjective (psychiatry)

Denoting a form of neurosis in which the sufferer is driven to perform a particular act repeatedly