In biology, irritability refers to the ability of living organisms to respond to a stimulus that tends to disturb their steady state or homeostasis. This can be exhibited by certain cells, such as muscle cells, neurons, and endocrine cells, which receive and respond to a stimulus. Irritability can be demonstrated in behavioral responses to both physiological and behavioral stimuli, including environmental, situational, sociological, and emotional stimuli.

Irritability can also refer to the pathological, abnormal, or excessive sensitivity to stimuli. In humans, irritability may be a significant transdiagnostic symptom or disposition that occurs across or at any point during the lifespan. It is commonly defined as the tendency to react to stimuli with the experience of negative affective states (especially anger) and temper outbursts, which may or may not be aggressive.

In simple organisms, such as algae, protozoans, and fungi, a response in which the organism moves toward or away from the stimulus is called taxis. In larger and more complicated organisms, the response involves the synchronization and integration of events in different parts.

Overall, irritability is an important characteristic of living organisms that allows them to respond to changes in their environment and maintain homeostasis.