Nancy has a headache; she takes some aspirin, and the headache goes away. Nancy is more likely to take aspirin again. This is an example of

The situation is an example of negative reinforcement.

In the condition of negative reinforcement, a stimulus especially an unpleasant stimulus is removed due to a particular behavior of the person. In this case, the unpleasant behavior is the headache, and taking aspirin is a behavior of Nancy, which cut of the stimuli, that is pain relieving. In negative reinforcement a particular behavior is increased, in order to remove the unpleasant stimulus.