Obsessive-compulsive disorder (OCD) is an anxiety disorder characterized by recurrent and disturbing thoughts (called obsessions) and/or repetitive, ritualized behaviours that the person feels driven to perform (called compulsions). Obsessions can also take the form of intrusive images or unwanted impulses. The majority of people with OCD have both obsessions and compulsions, but a minority) have obsessions alone or compulsions alone.

Common types of obsessions include concerns with contamination (e.g., fear of dirt, germs or illness), safety/harm (e.g., being responsible for a fire), unwanted acts of aggression (e.g., unwanted impulse to harm a loved one), unacceptable sexual or religious thoughts (e.g., sacrilegious images of Christ) and the need for symmetry or exactness.

 Common compulsions include excessive cleaning (e.g., ritualized hand washing); checking, ordering and arranging rituals; counting; repeating routine activities (e.g., going in/out of a doorway) and hoarding (e.g., collecting useless items). While most compulsions are observable behaviours (e.g., hand washing), some are performed as unobservable mental rituals (e.g., silent recitation of nonsense words to vanquish a horrific image).