It seems to me that there is a weak link in the chain of reasoning by which we explain experience and behavior, namely, our concept of the stimulus. The aim of this paper is to find out what psychologists mean by the term stimulus, with the hope of deciding what they ought to mean by it. After a short look at the history of the term, I will try to uncover the sources of confusion in modern usage. In the end, perhaps, the concept will be clarified. If not, certain contradictions will have been brought to light.