1.NounThe state or quality of being irritable; quick excitability.
"irritability of temper"
2.NounA natural susceptibility, characteristic of all living organisms, tissues, and cells, to the influence of certain stimuli, response being manifested in a variety of ways.
3.NounA condition of morbid excitability of an organ or part of the body; undue susceptibility to the influence of stimuli.