Overactive describes something that's working too hard, too fast, or too much. An overactive puppy bounces off the walls, unable to settle down even when everyone else is tired. An overactive imagination might convince you that every shadow in your bedroom is a monster, even though you know better.

The word often appears in medical contexts. Doctors might talk about an overactive thyroid, a gland that's producing too many hormones and making someone's heart race or hands shake. An overactive immune system might attack the body's own healthy cells by mistake, treating them like dangerous invaders.

In everyday life, overactive usually suggests something needs to calm down or work less intensely. A teacher might notice a student's overactive pencil tapping is distracting the whole class. Someone with an overactive sweet tooth might struggle to walk past a bakery without stopping. While being active is generally good, being overactive can create problems. It's the difference between a helpful level of energy and activity that's exhausting or disruptive to yourself or others.