Reactive means responding to something after it happens rather than preparing for it beforehand. When you're reactive, you wait until a problem appears before taking action. If you study only after failing a quiz, that's a reactive approach. If you fix your bike only after the chain breaks, you're being reactive.

The opposite of reactive is proactive: planning ahead and preventing problems before they occur. A reactive student starts their science project the night before it's due. A proactive student begins a week early. A reactive soccer goalie waits to see where the ball goes. A proactive goalie studies the shooter's habits and positions herself where the shot is likely to go.

Being reactive isn't always bad. Doctors practice reactive medicine when they treat illnesses as they appear. Fire departments respond reactively to emergencies. But constantly operating in reactive mode can be exhausting, like playing catch-up all the time. When you're always reacting, you're letting circumstances control you instead of taking charge of your situation.

In chemistry, reactive describes substances that change easily when mixed with other chemicals. Sodium is highly reactive: drop it in water and it fizzes and sparks dramatically. This scientific meaning shares the same basic idea: responding quickly and dramatically to outside influences.