The Collaborative International Dictionary of English v.0.48:

Irritant \Ir"ri*tant\, n. [Cf. F. irritant.]
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   1. That which irritates or excites.
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   2. (Physiol. & Med.) Any agent by which irritation is
      produced; as, a chemical irritant; a mechanical or
      electrical irritant.
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   3. (Toxicology) A poison that produces inflammation.
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   Counter irritant. See under Counter.

   Pure irritant (Toxicology), a poison that produces
      inflammation without any corrosive action upon the
      tissues.
      [1913 Webster]