Define stimuli.

A stimulus is any change in the environment that can be detected and causes a response in an organism or a cell.

In biology, “stimuli” is the plural of stimulus. A stimulus can be external, such as light, sound, temperature, touch, or chemicals, or internal, such as blood glucose level or hormone signals. Receptors detect stimuli and send signals through nerves or cell pathways to produce responses. In plants, stimuli cause tropic movements, like phototropism toward light.

In animals, stimuli drive reflexes and behaviors, such as withdrawing a hand from a hot object. In cell biology, ligands binding to receptors are stimuli that trigger signaling cascades. This concept links to homeostasis, nervous system function, and plant physiology.

Examples

• Eyes detect light; pupils constrict in bright light.
• Skin detects heat; you pull back from hot surfaces.
• Plant shoots bend toward sunlight.
• Insulin release responds to high blood sugar.