Damned if I know what it is, but according to the American Psychiatry Association:
Attention-deficit/hyperactivity disorder (ADHD) is one of the most common mental disorders affecting children. Symptoms of ADHD include inattention (not being able to keep focus), hyperactivity (excess movement that is not fitting to the setting) and impulsivity (hasty acts that occur in the moment without thought). ADHD is considered a chronic and debilitating disorder and is known to impact the individual in many aspects of their life including academic and professional achievements, interpersonal relationships, and daily functioning (Harpin, 2005).
ADHD became a "thing" in the 1960s. I first heard about it in 1969 when I was working at Boston State Mental Hospital; there was an investigation going on into whether giving stimulants to hyperactive children would work. At the time there were a batch of theories about why some children were hyperactive: eating too much sugar, too many soft drinks with caffeine, food dyes, too much television, chaotic home life, bad schools, and so on. Paradoxically, some stimulants acted like sedatives in pre-pubescent children.
Some of us (young idealistic radical types) at the hospital thought that hyperactivity was a diagnosis of black boys who didn't behave in school. We also thought it unlikely that wealthy white suburban boys would be given these drugs. But, what did we know?
55 years later, ADHD has become part of the furniture. There is still doubt about it. I notice that quite a few older people I know (i.e., people in their 70s) are good at sitting still, but they display a marked level of inattention. Their attention is scattered and their ability to concentrate is limited, yet they don't seem to have dementia. Dementia, of course, would degrade one's attention, concentration, memory, and other cognitive processes.
I wonder if adult attention deficits are often the result of poor mental hygiene -- that is, they have not recently disciplined themselves to pay attention, concentrate, remember important facts, and so on.
I also wonder if attention deficits are often the result of depression.
Quite a few people think the problem with young boys is that they are in schools where there are far too few opportunities to engage in uninhibited movement. Apparently many schools do not have recess periods where children are let loose to play outside or in a gym.
Then there is variability of behavior: is hyperactivity designed to account for children who don't fit into th desired mold?