Dramatic irony is irony inherent in the speeches or situations in which the characters find themselves and the irony is understood by the audience, but the characters themselves are unaware thereof. Simply put, the audience knows things which the characters do not, and they act or say things without realising the irony of what they say or do.
A good example of this is when the second witch greets Macbeth thus:
All hail, Macbeth, hail to thee, thane of Cawdor!
Macbeth's response is:
Stay, you imperfect speakers, tell me more:
By Sinel's death I know I am thane of Glamis;
But how of Cawdor? the thane of Cawdor lives,
A prosperous gentleman; and to be king
Stands not within the prospect of belief,
No more than to be Cawdor.
Macbeth expresses doubt about the fact that the witches greet him by such a noble title. He knows that he is Thane of Glamis, but how could he possibly be thane of Cawdor when he is still alive, a wealthy gentleman? To be thane of Cawdor is just as much beyond belief as to believe...
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