Irritability is the excitatory ability that living organism 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. Neurobiologically, irritability can be adaptive and productive, motivating the living organism to overcome barriers, and it can be non-productive, resulting in maladaptive behavior, when prolonged or disproportionate. The underlying brain circuits are thought to be those that are involved with reward processing.
In humans, irritability can be viewed as the consequence of a low threshold for experiencing frustration or "frustrative nonreward", when one is failing to receive an expected reward. There is no standard definition and there are no standard measurements in psychiatry, which would be spanning childhood through adulthood.