The Collaborative International Dictionary of English v.0.48:

Irony \I"ron*y\, n. [L. ironia, Gr. ? dissimulation, fr. ? a
   dissembler in speech, fr. ? to speak; perh. akin to E. word:
   cf. F. ironie.]
   [1913 Webster]
   1. Dissimulation; ignorance feigned for the purpose of
      confounding or provoking an antagonist.
      [1913 Webster]

   2. A sort of humor, ridicule, or light sarcasm, which adopts
      a mode of speech the meaning of which is contrary to the
      literal sense of the words.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Irony \I"ron*y\, a. [From Iron.]
   [1913 Webster]
   1. Made or consisting of iron; partaking of iron; iron; as,
      irony chains; irony particles; -- In this sense iron is
      the more common term. [R.] --Woodward.
      [1913 Webster +PJC]

   2. Resembling iron in taste, hardness, or other physical
      property.
      [1913 Webster]

WordNet (r) 3.0 (2006):

irony
    n 1: witty language used to convey insults or scorn; "he used
         sarcasm to upset his opponent"; "irony is wasted on the
         stupid"; "Satire is a sort of glass, wherein beholders do
         generally discover everybody's face but their own"--
         Jonathan Swift [syn: sarcasm, irony, satire, caustic
         remark]
    2: incongruity between what might be expected and what actually
       occurs; "the irony of Ireland's copying the nation she most
       hated"
    3: a trope that involves incongruity between what is expected
       and what occurs

Moby Thesaurus II by Grady Ward, 1.0:

76 Moby Thesaurus words for "irony":
   Atticism, Janus, agile wit, ambiguity, ambiguousness, ambivalence,
   amphibology, antinomy, biformity, bifurcation, black humor,
   burlesque, caricature, causticity, comedy, complexity of meaning,
   conjugation, cynicism, dichotomy, double entendre, double meaning,
   double reference, doubleness, doublethink, doubling, dry wit,
   dualism, duality, duplexity, duplication, duplicity, equivocacy,
   equivocality, equivocalness, equivocation, esprit, farce, halving,
   humor, innuendo, invective, lampoon, levels of meaning,
   multivocality, nimble wit, oxymoron, pairing, paradox, parody,
   paronomasia, pleasantry, polarity, polysemousness, polysemy,
   pretty wit, punning, quick wit, ready wit, richness of meaning,
   salt, sarcasm, satire, satiric wit, savor of wit,
   self-contradiction, slapstick, slapstick humor, squib, subtle wit,
   travesty, twinning, two-facedness, twoness, uncertainty,
   visual humor, wit



Bouvier's Law Dictionary, Revised 6th Ed (1856):

IRONY, rhetoric. A term derived from the Greek, which signifies
dissimulation. It is a refined species of ridicule, which, under the mask of
honest simplicity or ignorance, exposes the faults and errors of others, by
seeming to adopt or defend them.
     2. In libels, irony may convey imputations more effectually than direct
assertion, and render the publication libelous. Hob. 215; Hawk. B. 1, c. 73,
s. 4; 3 Chit. Cr. Law, 869, Bac. Ab. Libel, A 3.