the state or quality of being irritable; quick excitability; petulance; fretfulness; as, irritability of temper.-a natural susceptibility, characteristic of all living organisms, tissues, and cells, to the influence of certain stimuli, response being manifested in a variety of ways, -- as that quality in plants by which they exhibit motion under suitable stimulation; esp., the property which living muscle processes, of responding either to a direct stimulus of its substance, or to the stimulating influence of its nerve fibers, the response being indicated by a change of form, or contraction; contractility.-a condition of morbid excitability of an organ or part of the body; undue susceptibility to the influence of stimuli. see irritation, n., 3.