What is an event or action that makes organisms respond called?

An event or action that makes organisms respond is called a stimulus. A stimulus can be any change in the environment, such as a sound, light, touch, or chemical that triggers a response in an organism. The response can be a behavior, a physiological change, or a combination of both. For example, the sound of a predator can be a stimulus that triggers the response of running away or hiding in prey animals.