Had to look it up on Wikipedia:

'Irony (from the Ancient Greek εἰρωνεία eironeía, meaning hypocrisy, deception, or feigned ignorance) is a literary or rhetorical device, in which there is an incongruity or discordance between what one says or does and what one means or what is generally understood. Irony is a mode of expression that calls attention to the character's knowledge and that of the audience.

There is some argument about what qualifies as ironic, but all senses of irony revolve around the perceived notion of an incongruity between what is said and what is meant; or between an understanding of reality, or an expectation of a reality, and what actually happens.'