1. The state or quality of being irritable; quick excitability;
petulance; fretfulness; as, irritability of temper.
2. (Physiol.) 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 possesses, 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.
3. (Med.) A condition of morbid excitability of an organ or part of the
body; undue susceptibility to the influence of stimuli. See Irritation,
n., 3.