What is a stimulus and a response to it?

Stimulus, in general, is an event or thing that can cause or trigger a precise functional response in a tissue or organ. Also used in Psychology, it is a modification to the environment that can be responded and recorded by the senses. An example of this is how sound and light trigger your hearing and your vision respectively. It is often used as a measure to how a response is observed by a test subject, people or even organisms, in experiments.