The main characteristic of Obsessive-Compulsive Disorder (OCD) is the presence of obsessions and compulsions. Obsessions are "recurrent and persistent thoughts, urges, or images that are experienced as intrusive and unwanted." Compulsions are "repetitive behaviors or mental acts that an individual feels driven to perform in response to an obsession or according to rules that must be applied rigidly." Obsessions and compulsions take up a significant amount of time out of a person's day (an hour or longer).
DSM-IV Classification: 300.3
ICD-10-CM Classification: F42
(Definition from the Diagnostic and statistical manual of mental disorders: DSM-5)
(Statistics from the Gale Encyclopedia of Mental Health, 3rd edition)