A Stimulus (plural stimuli) is something which causes a response. It is used in various bodies:

• Stimulus (psychology): an input to a person's (or animal's) senses which causes a reaction or response. Especially used in classical conditioning and other kinds of behavioral experiments.
• Stimulus (physiology): a detectable change around an organism. Sensory receptors may then produce changes in the body's physiology.
• Input to a digital system. A stimulus is often the first component of a homeostatic control system.
• Stimulus (economics): a code-word for using tax and credit changes to boost a country's economy. Includes other measures to increase the supply of money.