Irony (per the OED)

1. (a)

A figure of speech in which the intended meaning is the opposite of that expressed by the words used; usually taking the form of sarcasm or ridicule in which laudatory expressions are used to imply condemnation or contempt.

 

1. (b)

with an and pl. An instance of this; an ironical utterance or expression.

 

2. 

fig. A condition of affairs or events of a character opposite to what was, or might naturally be, expected; a contradictory outcome of events as if in mockery of the promise and fitness of things. 

 

3.

In etymological sense: Dissimulation, pretence; esp. in reference to the dissimulation of ignorance practised by Socrates as a means of confuting an adversary ( Socratic irony).

 

4. (draft addition, 1993)

spec. in Theatr. (freq. as dramatic or tragic irony ), the incongruity created when the (tragic) significance of a character's speech or actions is revealed to the audience but unknown to the character concerned; the literary device so used, orig. in Greek tragedy.