Susceptibility of excitement; the quality of being easily irritated or exasperated; as irritability of temper.

In physiology, one of the four faculties of the sensorium, by which fibrous contractions are caused in consequence of the irritations excited by external bodies. Irritability differs from sensibility; the most irritable parts of the body not being at all sensible, and vice versa. The heart is endued with the greatest irritability.