Final answer:
A stimulus is a change or signal in the environment that can trigger an organism to react, often resulting in a response such as movement or physiological changes.
Step-by-step explanation:
A change or signal in the environment that can make an organism react is known as a stimulus. Organisms use their senses to detect stimuli and usually produce a response or reaction that can involve movement or other actions, such as a plant bending toward light (phototropism) or bacteria moving toward or away from chemicals (chemotaxis). For example, when an organism encounters a stimulus that causes an increase in body temperature, the responses may include sweating and the dilation of blood vessels in the skin to decrease the body temperature.