The word "chiasmus" comes from the Greek word for "cross." It refers to a rhetorical or literary device in which the grammatical structure of the second clause reverses that of the first, making it the opposite of grammatical parallelism. A parallel structure would be AB, AB, whereas the structure of a chiasmus is AB, BA. Take this example from Paradise Lost:

Love without end, and without measure Grace.

Here, the parallel structure would be "Love without end, and Grace without measure." Milton's line is much stronger, since both "Love" and "Grace" are emphasized by their positions at the beginning and end of the line, creating a perfect balance between the two, while enclosing the qualifying "without end" and "without measure" in the middle of the line.

Chiasmus is found in Shakespeare and Milton, but it is in the eighteenth century that it appears in most profusion, since this was an era in which both poets and prose writers were obsessed with balance and classicism. Take one of the most famous sentences in Gibbon's magisterial History of the Decline and Fall of the Roman Empire,

To resist was fatal, and it was impossible to fly.

Or, in verse, take Samuel Johnson's line from his Juvenalian satire, "The Vanity of Human Wishes,"

By day the frolic, and the dance by night.

The word "chiasmus" is often used to describe all such crossed structures, but purists will still insist that it should only be used to refer to a grammatical crossing without repeated words. Such phrases as the following are often called chiasmus.

Never let a fool kiss You or a kiss fool you.

But the repetition of the words "fool" and "kiss" mean that this is, strictly speaking, an antimetabole.