\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.