According to the Diagnostic and statistical manual of mental disorders (DSM 5), Body Dysmorphic Disorder is characterized by "preoccupied with one or more perceived defects or flaws in their physical appearance, which they believe look ugly, unattractive, abnormal, or deformed." These flaws are not apparent or only slightly apparent to other individuals. These preoccupations take up multiple hours a day. Individuals will have "excessive repetitive behaviors or mental acts (e.g., comparing) are performed in response to the preoccupation," which are hard to control.
DSM IV: 300.7
(Statistics from the Diagnostic and statistical manual of mental disorders (DSM 5))