Sam’s wife always wears the same black nightgown whenever she is “in the mood” for outing. Sam becomes aroused to go out as soon as he sees his wife in the nightgown. For Sam, the nightgown is a(n):

The nightgown is a conditioned stimulus because it was initially neutral but, through repeated association with "outing" (unconditioned stimulus), it now triggers Sam's arousal (conditioned response). It's a learned signal.