Irritability is an excessive response to stimuli. Irritability takes many forms, from the contraction of a unicellular organism when touched, to complex reactions involving all the senses of higher animals. In plants, response is usually different from that seen in animals, but is nonetheless present. The term irritability is used for both the physiological reaction to stimuli and for the pathological, abnormal or excessive sensitivity to stimuli.

Irritability may manifest in behavioral responses to both physiological as well as behavioral stimuli, including environmental, situational, sociological, and emotional stimuli.