An incrongruity between what is said and what is understood, or what is expected and what actually occurs. Irony can be used intentionally or can happen unintentionally.
Example From Text
The world will little note, nor long remember what we say here...
This is an example of unintentional irony. Lincoln stated and believed that few people would remember the words of this speech. Therefore, it is ironic that this speech has turned out to be one of the most famous and most memorable speeches in American history.