ADHD is ...

... "an inherited neurological sydrome characterized by easy distractibility, low tolerance for frustration or boredom, and a greater than average tendency to say or do whatever comes to mind." (Daniel Amen, MD 2002)

... "a fundamental deficit in self-control or self-regulation with attention problems a secondary characteristic." (Russell Barkley, PhD 1994)

... "an inherited neurobiological difference which results in processing deficits related to self-regulation and academic or vocational performance and is often accompanied by difficult and disruptive behavior." (Mind Games working definition 2008)

ADHD is ...

...  recognized by bundles of behavioral symptoms of inattention, hyperactivity, or impulsivity.

... probably a difference in brain structure. Definitely a difference in processing capability in the areas of  "executive function" and self-regulation of behavior.

...  found in boys, girls, and adults of both genders and all ethnicities.

... significantly affects performance at school or work.

... often accompanied by other disorders - "co-morbid" conditions - such as Oppositional Defiant Disorder, Conduct Disorder, Anxiety, or Depression.

... probably inherited. ADHD runs in families.

... is a lifetime condition. Individuals do not "outgrow" ADHD although some symptoms may moderate into adolescence and adulthood.