What does "irritability" mean?

Irritability: (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 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.