Source : Webster's Revised Unabridged Dictionary (1913)

Irritability \Ir`ri*ta*bil"i*ty\, n. [L. irritabilitas: cf. F.
   irritabilit['e].]
   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 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.

   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.

Source : WordNet®

irritability
     n 1: an irritable petulant feeling [syn: {crossness}, {fretfulness},
           {fussiness}, {peevishness}, {petulance}, {choler}]
     2: excessive sensitivity of an organ or body part [syn: {excitability}]
     3: a disposition to exhibit uncontrolled anger; "his temper was
        well known to all his employees" [syn: {temper}, {biliousness},
         {peevishness}, {pettishness}, {snappishness}, {surliness}]