Stimulus is an external or internal event or condition that provokes a response in an organism. It can be any change in the environment, a change in temperature, light, sound, pressure or touch. It can also be a change in the internal state of an organism, such as a change in hormone levels, pH, or nutrient levels.

Explanation:

Stimulus is a term used in biology to describe any change in the environment or internal state of an organism that produces a response. The response can be a behavior, a physiological change, or a biochemical reaction. The stimulus can be physical, chemical, or biological in nature.

Stimuli can be classified as external or internal. External stimuli come from the environment, such as changes in temperature, light, sound, pressure, or touch. Internal stimuli arise from changes within an organism, such as changes in hormone levels, pH, or nutrient levels.

Examples of stimuli and responses:

- A loud noise (stimulus) causes an individual to jump (response).
- A sudden drop in temperature (stimulus) causes an organism to shiver (response).
- A change in the level of glucose in the blood (stimulus) causes the pancreas to release insulin (response).
- A change in pH (stimulus) causes an enzyme to denature (response).

In conclusion, anything that produces a response in an organism is called a stimulus. Stimuli can be external or internal and can result in a wide range of responses.

Anything that gives a response is called stimulus