When trying to define anything, it's usually pretty prudent to start with a dictionary definition and then discuss what it means and how ironic situations generally follow the so-called official rules. So:
- The use of words to express something different from and often opposite to their literal meaning.
- An expression or utterance marked by a deliberate contrast between apparent and intended meaning.
- A literary style employing such contrasts for humorous or rhetorical effect.
(taken from The American HeritageŽ Dictionary of the English Language, Fourth Edition)
- Incongruity between what might be expected and what actually occurs
- An occurrence, result, or circumstance notable for such incongruity.
An explanation that just barely captures the true feeling and definition of irony. The definitions taken from the dictionary, as you can see, are confusing - and that's not what a definition is supposed to be! The first two explanations from the first set of definitions above, appear to be actually a mixture of sarcasm and tragic irony. The second set of definitions seem to have been added to balance what irony actually is, with what it is most often mistaken for. You see? Not necessarily very helpful.
It's also not necessarily anyone's fault that it is difficult to pin down an accurate "cover all" definition of irony - it is instead due to two "problems" regarding the evolution of language:
Regarding the first in the list: irony actually takes on many literary and completely genuine forms. With so many different definitions of irony, it is understandable that a "cover all" definition does not exist, and also that the dictionary's definition contains so many variations of meaning.
Regarding the second, above: the term "irony" is often misused. People will tend misuse irony to describe events which are simply unfortunate rather than ironic. For example:
It is ironic that Sir Alec Guinness is best known for his work in George Lucas' Star Wars movies, even though he himself disliked them?
According to standard definitions, this is not an ironic situation. Increasingly, this misuse of irony is becoming accepted as a genuine secondary explanation of irony, which is not necessarily something to be abhorred but it is important to be aware of it. However, since the term is misused so often, a form of irony called "Situational Irony" or "Cosmic Irony" has come into being. This is, basically, used to describe situations when God or Fate is playing silly buggers.