A biologic stimulus is any external change in the environment that can be detected by an organism. The ability to respond to a stimulus is called irritability and is a necessary condition for life.
An example of a stimulus is an individual placing a hand on a hot surface. The temperature is the stimulus, and the individual then protects themselves by pulling their hand away. Other stimuli include the ability of an organism to detect the pH, salinity or light in their environment. All organisms must maintain homeostasis, a stable internal environment. Homeostasis can only be achieved through sensing and responding to the environment.