Irritability is the excitatory ability that living organisms have to respond to changes in their environment. The term is used for both the physiological reaction to stimuli and for the pathological, abnormal or excessive sensitivity to stimuli. It is usually used to refer to anger or frustration. In individuals with autism disorder for example, they tend to be marked by aggression patterns. Irritability can be a growing response to the objective stimuli of hunger or thirst in animals or humans which then reaches some level of awareness of that need.
Irritability may be demonstrated in behavioral responses to both physiological and behavioral stimuli including environmental, situational, sociological, and emotional stimuli.