v. t. [ L. obsessus, p. p. of obsidere to besiege; ob (see Ob-) + sedere to sit. ] 1. To besiege; to beset. [ archaic ] Sir T. Elyot. [ 1913 Webster ]

2. To excessively preoccupy the thoughts or feelings of; to haunt the mind persistently. [ PJC ]

v. i. To be excessively or persistently preoccupied with something; -- usually used with on or over; as, to obsess over an imagined insult. [ PJC ]

At all ages children are driven to figure out what it takes to succeed among their peers and to give these strategies precedence over anything their parents foist on them. Weary parents know they are no match for a child's peers, and rightly obsess over the best neighborhood in which to bring their children up. Steven Pinker (How the Mind Works, p. 449-450 [ 1997 ]). [ PJC ]

adj. 1. having or showing excessive or compulsive concern; -- used with with.
Syn. -- haunted, preoccupied, taken up(predicate). [ WordNet 1.5 ]

2. influenced or controlled by a powerful force such as a strong emotion.
Syn. -- possessed(predicate). [ WordNet 1.5 ]

n. [ L. obsessio: cf. F. obsession. ] 1. The act of besieging. [ archaic ] Johnson. [ 1913 Webster ]

2. The state of being besieged; -- used specifically of a person beset by a spirit from without. [ archaic ] Tylor. [ 1913 Webster ]

Whether by obsession or possession, I will not determine. Burton. [ 1913 Webster ]

3. An excessive preoccupation of the thoughts or feelings; the persistent haunting or domination of the mind by a particular desire, idea, or image. [ PJC ]

4. Hence: Any driving motive; a compelling goal; -- not necessarily implying a negative judgment, as does sense 3; as, the coach was obsessed with winning the state championship [ PJC ]

5. Something that causes an obsession{ 3 }. [ PJC ]

6. The state of being obsessed. [ PJC ]

adj. Persistently and abnormally preoccupied with some unreasonable idea.
Syn. -- compulsive. [ WordNet 1.5 ]