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: Webster's Revised Unabridged Dictionary (1913)