Chiasmus (key-AZ-mus) is a figure of speech in which the order of particular words in the first of two parallel phrases is reversed in the second. Chiasmus was most popular in 18th century English poetry but is also found in other eras such as the 1960's with JFK's "Ask not what your country can do for you, but what you can do for your country."

The term chiasmus comes from the Greek word khiasmos meaning "crossing", which came from chiazein. Chizein means "to mark with a chi, X", indicating the crossed arrangement of terms. To determine whether a phrase is chiastic, it should be laid out in parallel like this and see if an X can be drawn. If not, the phrase is probably not chiastic.:

"You must master your rage          
              \        /
               \      /
                \    /
                 \  /
                  \/
                  /\
                 /  \
                /    \
               /      \
              /        \
 before your rage masters you."  
(a quote from the Mystery Men character "The Sphinx" speaking to "Mr. Furious")

"Never let a fool kiss you
                \  /
                 \/
                 /\
                /  \ 
        or a kiss fool you."
(a quote from a chiastic expert, Dr. Mardy Grothe)

   "To finish first, 
            \  /
             \/
             /\
            /  \
one must first finish."
"Quitters never win. Winners never quit. But those who never win and never quit are idiots."
(from http://www.despair.com)