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