Essentially, there is none. In 1987, attention deficit disorder (ADD) was renamed to attention deficit hyperactivity disorder (ADHD) to best represent the disorder’s hyperactive symptoms. Though the symptoms remain the same, people diagnosed with ADHD are further divided into three subtypes: predominantly inattentive, predominantly hyperactive/impulsive or combined. Individuals whose symptoms fall mostly under the category of predominantly hyperactive/impulsive would be diagnosed as “ADHD, subtype hyperactive/impulsive whereas individuals whose symptoms fall mostly under the category of predominantly inattentive would receive the diagnosis of “ADHD, subtype inattentive.” Thus, the term ADD no longer exists as an official diagnosis. However, ADD is often used when referring to the disorder in general.
Did you know that the American Psychology Association is considering going back to calling the disorder ADD? The association may revise the name once again for the fifth edition of the Diagnostic Statistical Manual of Mental Disorders due out in 2013.