Often wrongly named "Helsinki-syndrome".
The Stockholm syndrome describes a psychological process which appears to be absurd at first notion: victims of a kidnapping or people who are taking hostage build up a positive relation to their kidnappers, they solidarize with the very people responsible for their distress. To them their potential rescuer, i.e. the police, are more dangerous than their kidnappers as they hinder the kidnapper to escape. In extreme cases victims fall in love with the criminals, feel pity for them and support them. The longer hostages are held captive the more likely they will sympathize with the hostage-takers.
The syndrome is named after an incident that occured in Stockholm on April 23rd, 1973, when a young man attempted to rob a bank and took four people hostage. The enormous media attention was new at the time with TV reporting from the scene, displaying openly the fear of the hostages. These seemed to be more afraid of a potential police action than of the young man - a phenomenon that was thought never to have been witnessed before.