Here's a pretty good definition, though incomplete:

"Irony--originally a deceptive form of understatement (from the Greek eiron, a stock comic character who typically equivocated, misled his listeners, or concealed complex meanings behind seemingly simple words); hence an attribute of statements in which the meaning is different--or more complicated--than it seems. A subtle form of sarcasm, verbal irony is a rhetorical device in which the speaker either severely understates his point or means the opposite of what he says (as when a guest politely describes a host's unimpressive wine as "nicely chilled" or a conspicuously dull person is described as "not a likely Mensa candidate."Dramatic irony arises in situations where two or more individuals have different levels of understanding or different points of view. More specifically, it occurs when the audience or certain characters in a play know something that another character does not--as when Oedipus, ignorant that he himself is the person he seeks, vows to track down Laius's killer."

http://condor.depaul.edu/~dsimpson/awtech/lexicon.html

Best regards,
Dub