An aftereffect is something that happens as a result of an action. For example, an aftereffect of taking some kinds of allergy medication is feeling very sleepy.
While many aftereffects can essentially be side effects of taking some drug or medicine, there are many other types of aftereffects. An aftereffect of being rude to a customer at the restaurant where you work might be a very small tip, and the aftereffects of being robbed could include a fear of walking alone at night. The word combines the Old English root æfter, "after, next, or later," and the Latin effectus, "accomplishment or result."