Imagine that you flinch after seeing lightning because in previous instances the lightning is followed by thunder, which scared you. in this scenario, lightning can be interpreted as being a(n):

The answer is a conditioned stimulus. In this case, the lightning did not have a response from you attached to it until the thunder followed it. Once thunder, a loud and unpleasant noise, surprised you, the lightning now has a new meaning. It now alerts you that something is to follow, and so it has changed from a neutral stimulus (one with no attachment to it) to a conditioned stimulus.