What is chiasmus?

Chiasmus is a balancing pattern in which the second part of a line reverses and balances the word in the first part. It is a form of contrast deliberately built. By this figure the order of words in the first of two parallel clauses or phrases is reversed in the second which serves to make a sentence more emphatic and impressive. Sometimes all the words in the first part of a sentence are reversed in the second part for the sake of contrasting ideas. 

Thus in the example “Fair is foul and foul is fair”, the words of the first part are reversed in the second part for the sake of drawing our attention to the contrast that lies between the first idea and the last. Here the order of words in Fair is foul is reversed into foul is fair to point out the contrast lying between the two ideas. This inversion aims to make the statement more emphatic and impressive.