Obsessive compulsive disorder (OCD) is a mental health condition where a person has obsessive thoughts and compulsive behaviour.
An obsession is an unwanted, unpleasant thought, image or urge that repeatedly enters a person's mind, causing them anxiety.
The word "obsession" usually describes something enjoyable, but in OCD the obsession is unpleasant and frightening.
A compulsion is a repetitive behaviour or mental act that someone feels they need to carry out to try to prevent an obsession coming true.
For example, someone who is obsessively scared they will catch a disease may have the compulsion to wash their hands every time they touch something.
Research shows that many people on the autism spectrum have obsessive compulsions.