From the OED (Oxford English Dictionary)
irony1
noun (plural ironies) the expression of meaning through the use of language signifying the opposite, typically for humorous effect. Øa state of affairs that appears perversely contrary to what one expects. Ø(also dramatic or tragic irony) a literary technique, originally used in Greek tragedy, by which the significance of a character's words or actions are clear to the audience or reader although unknown to the character.
ORIGIN
C16: via Latin from Greek eironeia 'simulated ignorance', from eiron 'dissembler'.