Everything about ADHD seems to cause confusion, including the name—until you get the facts, so clearly presented in Is it You, Me, or Adult A.D.D.? Stopping the Roller Coaster When Someone You Love Has Attention Deficit Disorder.
Until recently, the official term was ADD, plus or minus Hyperactivity, and that’s how many people still refer to it (hence the book title).
The new official term, Attention-Deficit/Hyperactivity Disorder (AD/HD), uses a slash mark to indicate that hyperactivity is sometimes but not always present.
In fact, hyperactivity is rarely present in adults. The physical restlessness common to children with ADHD sort of goes “underground” as the person matures, becoming more of a mentally restless state. (By the way, most books on the subject omit the slash for readability, so it’s ADHD.)