obsession

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Compulsive preoccupation with a fixed idea or an unwanted feeling or emotion, often accompanied by symptoms of anxiety.
• noun A compulsive, often unreasonable idea or emotion.

from The Century Dictionary.

• noun The act of besieging; persistent assault.
• noun Continuous or persevering effort supposed to be made by an evil spirit to obtain mastery of a person; the state or condition of a person so vexed or beset: distinguished from possession, or control by a demon from within.
• noun In pathology, a constant brooding upon any subject, such as the thought of death, until the mind becomes dominated by that one idea.

from the GNU version of the Collaborative International Dictionary of English.

• noun archaic The act of besieging.
• noun archaic The state of being besieged; -- used specifically of a person beset by a spirit from without.
• noun An excessive preoccupation of the thoughts or feelings; the persistent haunting or domination of the mind by a particular desire, idea, or image.
• noun Any driving motive; a compelling goal; -- not necessarily implying a negative judgment, as does sense 3
• noun Something that causes an obsession{3}.
• noun The state of being obsessed.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A compulsive or irrational preoccupation.
• noun An unhealthy fixation.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an unhealthy and compulsive preoccupation with something or someone
• noun an irrational motive for performing trivial or repetitive actions, even against your will