Obsessive compulsive disorder is one of the anxiety disorders recognized in the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition. OCD is characterized by the presence of obsessions and compulsions.
Obsessions are very persistent thoughts that recur frequently and that cause the person significant anxiety or distress (e.g., “I left the front door unlocked.”) Compulsions are repetitive behaviors that the person engages in to try to address the obsession (e.g., checking the lock repeatedly).
Everyone occasionally has anxious thoughts come to mind and engages in some kind of behavior designed to reduce their anxiety about it. But people with OCD experience significant distress and often engage in compulsive behaviors to such a degree that it takes up a great deal of their time. Also, most people with OCD recognize that their obsessions and compulsions are excessive, but they feel powerless to stop the cycle.