This is the noun: the state, or the pattern, of being hyperactive. In ordinary talk it means too much restless energy — running, wriggling, not being able to settle. You can have a short burst of it after a party or a fizzy drink, and it fades when the energy fades. In school letters and medical talk the word is more serious: a longer difficulty with keeping still and, often, with attention. Friends almost never say 'he has hyperactivity' about a wild hour; they say 'he's so hyper'. You meet the longer word when adults are being careful and official. Only a doctor should treat the official use as a name for a condition. The everyday use is just a description of energy that will not sit down.