In the 1960's the major classification systems of the day, the Diagnostic and Statistical Manual of Mental Disorders (DSM-II) and the International Statistical Classification of Diseases and Related Health Problems (ICD-9) had more descriptive titles with emphasis on hyperactivity as the key symptom (ex: hyperactive child syndrome, hyperkinetic syndrome). As more research was conducted there was a general suggestion that hyperactivity was a secondary problem to impulse control and the inability to maintain attention. Therefore, in the DSM-III the condition was renamed "attention-deficit disorder". This disorder was described as consisting of three categories of symptoms: inattention, hyperactivity, and impulsivity. Three subtypes of attention-deficit disorder were also characterized; problems with attention with hyperactivity, without hyperactivity, and a third residual subtype for those who didn't meet full criteria for the other two diagnoses.
In 1987, the revised DSM-III (DSM-III-R) included the diagnosis attention deficit/hyperactivity disorder. This version was significantly different from the previous categorization, in particular in that this description did not include any subgroups. This diagnostic system was created by a committee of experts coming to a consensus. The most recent version of the diagnosis, the one currently in use in the DSM-IV, was defined in part by trials that involved children evaluated with interviews and diagnostic instruments created to test the validity of the different subgroups of ADHD.
What I hope is clear from this discussion is that ADHD is a diagnosis describing a collection of symptoms that have been generally recognized for over 100 years, but the specific requirements for the diagnosis have changed significantly. As recently as 1987 there was a very different set of rules governing the diagnosis of ADHD. It just may be that in the upcoming DSM-V, the rules may change again.