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 In pathology, a constant brooding upon any subject, such as the thought of death, until the mind becomes dominated by that one idea.
• 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.

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