An aftereffect is something that happens as a result of an earlier event, usually appearing some time after the original event is over. When you spin around in circles and then stop, the dizziness you feel is an aftereffect of the spinning. When a town experiences flooding after a hurricane has passed, that flooding is an aftereffect of the storm.

The word often describes consequences that linger or emerge gradually. Medicine can have aftereffects: an antibiotic might cure your infection but leave you feeling tired for a few days afterward. A scary movie might have aftereffects like nightmares or nervousness at bedtime. Historical events have aftereffects too: the aftereffects of a war might include changes to borders, economies, or how people think about the world.

Notice that aftereffects aren't always negative, though the word often appears when describing problems or challenges. A great concert might have the pleasant aftereffect of a melody stuck in your head for days. The aftereffects of hard training might include stronger muscles and better endurance.