Stimulus
Stimulus explained in an easy to understand way:
Imagine you're in a dark room, and suddenly someone flicks on the light switch, causing you to squint or shield your eyes. This situation is similar to the concept of a stimulus, which is anything that prompts a reaction from an organism. Just like the sudden brightness of the light causes you to respond by squinting, a stimulus triggers a response from an organism to adapt or react to changes in its environment.
Practice Version
Stimulus: Anything that causes an organism to react stimulus. A stimulus is something that makes a living thing respond or change in some way.