How well defined is ADD/ADHD?
Like all psychiatric disorers, ADD and ADHD are diagnosed using a set of criteria listed in the Diagnostic and Statistical Manual of Mental Disorders, or DSM

The latest version is the DSM-IV-TR. The DSM-V is due out in 2013 and may change these criteria.

Diagnosis is expected to be done by a licensed professional who is able to assess these criteria.

From http://www.mental-health-today.com/add/dsm.htm :

INATTENTION
(need 6 of 9)

- often fails to give close attention to details or makes careless mistakes in schoolwork, work or other activities
- often has difficulty sustaining attention in tasks or play activities
- often does not seem to listen when spoken to directly
- often does not follow through on instructions and fails to finish schoolwork, chores, or duties in the workplace (no if oppositional behavior or doesn’t understand instructions)
- often has difficulty organizing tasks and activities
- often avoids, dislikes, or is reluctant to engage in tasks or activities that require sustained mental effort (such as schoolwork or homework)
- often loses things necessary for tasks or activities (e.g., toys, school assignments, pencils, books, or tools)
- often easily distracted by extraneous stimuli
- often forgetful in daily activities

HYPERACTIVITY-IMPULSIVITY  
(need 6 of 9)

- often fidgets with hands or feet or squirms in seat
- often leaves seat in classroom or in other situations in which remaining seated is expected
- often runs about or climbs excessively in situations in which it is inappropriate (in adolescents or adults, may be limited to subjective feelings of restlessness)
- often has difficulty playing or engaging in leisure activities quietly
- is often "on the go" or often acts as if "driven by a motor"
- often talks excessively
- often blurts out answers before questions have been completed
- often has difficulty awaiting turn
- often interrupts or intrudes on others (e.g., butts into conversations or games)

REQUIREMENTS

- Present at least 6 months, maladaptive and inconsistent with development level
- Some symptoms that caused impairment were present before age 7
- Some impairment from the symptoms is present in two or more settings (e.g., at school {or work} and at home)
- There must be clear evidence of clinically significant impairment in social, academic or occupational functioning