Someone who is hyperactive is too active, and cannot keep still or be quiet for very long. The usual picture is a child who gets up, talks over people, starts many things, and whose body seems to have extra electricity. In everyday English, parents and teachers use it loosely — a noisy afternoon, a puppy, even an adult after too much coffee. In a clinic the same word is heavier: it may be part of a careful picture of how a child pays attention and controls their body, often together with the letters ADHD. Kind speakers are careful not to stick it on a child as a forever label. 'Hyperactive this morning' is about one stretch of time; 'a hyperactive child' said like a name can sting. It is not a rude word, and it is not praise either. Formal writing may prefer 'cannot keep still' or a doctor's term; everyday English is happy with hyperactive.