Stimulant refers to anything that temporarily arouses or speeds up mental or physical activity: "Alcohol, thought to be a stimulant, is actually a depressant." A stimulus is something that rouses to action, that incites activity, that produces a response: "Additional pay is a stimulus to harder work." The effect of a stimulant is short-lived; that of a stimulus does not necessarily involve time restrictions. The plural of stimulant is stimulants; that of stimulus is stimuli.
Dictionary of problem words and expressions. Harry Shaw. 1975.