Source : Webster's Revised Unabridged Dictionary (1913)

Irritation \Ir`ri*ta"tion\, n. [L. irritatio: cf. F.
   irritation.]
   1. The act of irritating, or exciting, or the state of being
      irritated; excitement; stimulation, usually of an undue
      and uncomfortable kind; especially, excitement of anger or
      passion; provocation; annoyance; anger.

            The whole body of the arts and sciences composes one
            vast machinery for the irritation and development of
            the human intellect.                  --De Quincey.

   2. (Physiol.) The act of exciting, or the condition of being
      excited to action, by stimulation; -- as, the condition of
      an organ of sense, when its nerve is affected by some
      external body; esp., the act of exciting muscle fibers to
      contraction, by artificial stimulation; as, the irritation
      of a motor nerve by electricity; also, the condition of a
      muscle and nerve, under such stimulation.

   3. (Med.) A condition of morbid excitability or
      oversensitiveness of an organ or part of the body; a state
      in which the application of ordinary stimuli produces pain
      or excessive or vitiated action.

Source : WordNet®

irritation
     n 1: the psychological state of being irritated or annoyed [syn:
          {annoyance}, {vexation}, {botheration}]
     2: a sudden outburst of anger; "his temper sparked like damp
        firewood" [syn: {pique}, {temper}]
     3: (pathology) abnormal sensitivity to stimulation; "any food
        produced irritation of the stomach"
     4: the neural or electrical arousal of an organ or muscle or
        gland [syn: {excitation}, {innervation}]
     5: an uncomfortable feeling in some part of the body [syn: {discomfort},
         {soreness}]
     6: unfriendly behavior that causes anger or resentment [syn: {aggravation},
         {provocation}]
     7: the act of troubling or annoying someone [syn: {annoyance},
        {annoying}, {vexation}]