Source : Webster's Revised Unabridged Dictionary (1913)

Irritant \Ir"ri*tant\, n. [Cf. F. irritant.]
   1. That which irritates or excites.

   2. (Physiol. & Med.) Any agent by which irritation is
      produced; as, a chemical irritant; a mechanical or
      electrical irritant.

   3. (Toxicology) A poison that produces inflammation.

   {Counter irritant}. See under {Counter}.

   {Pure irritant} (Toxicology), a poison that produces
      inflammation without any corrosive action upon the
      tissues.