There are three predominant core features associated with AD/HD these are:

• Excessive inattention
• Impulsivities
• Hyperactivity

Sir George Still noted these three core features as far back as 1902 and commented on them as follows.

• Inattention
  “a notable feature in many of these cases... is a quite
  abnormal incapacity for sustained attention. Both parents
  and school teachers have specially noted this feature
  as something unusual”
  
• Impulsiveness
  “The serious danger which these children constitute to
  both themselves and to society calls for active
  recognition”
  
• Hyperactivity
  “The degree of moral control which may be perfectly normal
  in a very young child, may be altogether below the average
  for a child a few years older.”