The Collaborative International Dictionary of English v.0.48:

Irritability \Ir`ri*ta*bil"i*ty\, n. [L. irritabilitas: cf. F.
   irritabilit['e].]
   [1913 Webster]
   1. The state or quality of being irritable; quick
      excitability; petulance; fretfulness; as, irritability of
      temper.
      [1913 Webster]

   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.
      [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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