What term describes a signal to which an organism responds?

The term that describes a signal to which an organism responds is "stimulus." In biological terms, a stimulus refers to any external or internal factor that causes a reaction from an organism. This can include light, sound, temperature changes, or chemical signals, among others.

When an organism detects a stimulus, it often leads to a specific response or behavior. For example, if a plant senses sunlight (the stimulus), it may grow toward the light (the response). Understanding this concept is crucial in fields like biology and ecology, as it highlights how organisms interact with their environments.

While other terms relate to organismal responses, none encapsulates the trigger aspect of the signal like "stimulus" does. Behavior refers to the actions taken by the organism in response, while cue is more often used in the context of environmental signals or indicators that help guide behavior, but they do not specifically denote the signal itself.