Attention deficit disorder (ADD) is one of the three subtypes of Attention-deficit hyperactivity disorder (ADHD). The term was formally changed in 1994 in the new Diagnostic and Statistical Manual of Mental Disorders, fourth edition (DSM-IV) to "ADHD predominantly inattentive" (ADHD-PI or ADHD-I), though the term attention deficit disorder is still widely used. ADD is similar to the other subtypes of ADHD in that it is characterized primarily by inattention, easy distractibility, disorganization, procrastination, and forgetfulness; where it differs is in lethargy - fatigue, and having fewer or no symptoms of hyperactivity or impulsiveness typical of the other ADHD subtypes. Different countries have used different ways of diagnosing ADD. In the UK, diagnosis is based on quite a narrow set of symptoms, and about 0.5–1% of children are thought to have attention or hyperactivity problems. The USA used a much broader definition of the term ADHD. As a result, up to 10% of children in the USA were described as having ADHD. Current estimates suggest that ADHD is present throughout the world in about 1–5% of the population. About five times more boys than girls are diagnosed with ADHD. Medications include two classes of drugs, stimulants and non-stimulants. Drugs for ADHD are divided into first-line medications and second-line medications. First-line medications include several of the stimulants, and tend to have a higher response rate and effect size than second-line medications. Although medication can help improve concentration, it does not cure ADD and the symptoms will come back once the medication stops.

Attention deficit hyperactivity disorder (ADHD) is a developmental disorder. It is characterized primarily by "the co-existence of attentional problems and hyperactivity, with each behavior occurring infrequently alone" and symptoms starting before seven years of age.

Die Aufmerksamkeitsdefizit-/Hyperaktivitätsstörung (ADHS), die auch als Aufmerksamkeitsdefizit-/Hyperaktivitätssyndrom oder Hyperkinetische Störung (HKS) bezeichnet wird, ist eine bereits im Kindesalter beginnende psychische Störung, die sich durch Probleme mit der Aufmerksamkeit sowie Impulsivität und häufig auch Hyperaktivität auszeichnet. Etwa drei bis zehn Prozent aller Kinder zeigen Symptome im Sinne einer ADHS. Jungen sind deutlich häufiger betroffen als Mädchen. Die Symptome können mit unterschiedlicher Ausprägung bis in das Erwachsenenalter hinein fortbestehen.

Le TDAH ou trouble déficit de l'attention/hyperactivité (en anglais Attention-deficit hyperactivity disorder, ADHD ou Attention-deficit disorder, ADD) est décrit comme un trouble comportemental qui pourrait être d'origine neurobiologique et parfois environnementale.
Ce « syndrome » est caractérisé par des états d'hyperactivité, des moments d'absence, des changements d'humeur, des problèmes de concentration et d'impulsivité. Sa détection et les soins à apporter font l'objet de nombreuses controverses.

ADHD (Attention-Deficit/Hyperactivity Disorder) är en neuropsykiatrisk funktionsnedsättning som utmärks av bristande uppmärksamhet och hyper- eller hypoaktivitet. Det finns tre varianter av ADHD: en som innebär att man har koncentrationssvårigheter och tenderar att vara inobservant på sin omgivning, en som innebär att man är hyperaktiv och impulsiv, och en som innebär att man har båda de tidigare nämnda sorternas svårigheter.

ADHD – skrót z ang. Attention Deficit Hyperactivity Disorder, czyli Zespół nadpobudliwości psychoruchowej, określany w literaturze jako zespół nadpobudliwości psychoruchowej z deficytem uwagi lub zaburzenia hiperkinetyczne.