A drug or other agent that prompts the activity of a body system or function. For example, the sight and smell of food stimulates salivation, and the rods and cones in the retina of the eye are stimulated by light. Another example is the use of amphetamines and caffeine to stimulate the central nervous system and make an individual more alert and active – or, if taken in excess, hyperactive. In treatment procedures, electrical stimulation may be used to bring muscles into action. Aromatics, spices and bitters are traditional stimulants of digestive processes.