The term that describes an organism's responsiveness to stimuli is irritability or responsiveness. This ability allows organisms to detect and respond to changes in their environment, which is essential for survival. Responses can be physiological, behavioral, or developmental. For example, plants may bend towards sunlight (phototropism), while animals might flee from danger. This responsiveness plays a crucial role in natural selection, as those organisms that effectively adapt to stimuli are more likely to survive and reproduce.