Source : Webster's Revised Unabridged Dictionary (1913)

Irritant \Ir"ri*tant\, a. [L. irritans, -antis, p. pr. of
   irritare: cf. F. irritant. See {Irritate} to excite.]
   Irritating; producing irritation or inflammation.

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.

Irritant \Ir"ri*tant\, a. [LL. irritants, -antis, p. pr. of
   irritare to make null, fr. L. irritus void; pref. ir- not +
   ratus established.] (Scots Law)
   Rendering null and void; conditionally invalidating.

         The states elected Harry, Duke of Anjou, for their
         king, with this clause irritant; that, if he did
         violate any part of his oath, the people should owe him
         no allegiance.                           --Hayward.

Source : WordNet®

irritant
     n : something that causes irritation and annoyance; "he's a
         thorn in my flesh" [syn: {thorn}]