Obsessions are recurrent thoughts that are usually not welcome and which can be upsetting. Sometimes such obsessional thoughts become linked to recurrent habits or compulsions. These then become the pattern of how we cope to avoid getting very anxious.
For example, the recurrent thought that the house could be burgled can lead to the repetitive checking of doors to see if they are locked, so that leaving the house becomes a lengthy process of checking and rechecking until our anxiety has settled.
Often the fear that we are trying to control is out of all proportion to the risk, which makes this condition irrational and an illness.
Treatment is usually with psychological therapy and/or medication.