Chiasmus is
a figure of speech by which the order of the terms in the first of two
parallel clauses is reversed in the second. This may involve a repetition of
the same words, in which case the figure may be classified as antimetabole,
or just a reversed parallel between two corresponding pairs of ideas, as in
this line from Mary Leapor's 'Essay on Woman' (1751): Despised, if ugly; if
she's fair, betrayed.
The figure
is especially common in 18th-century English poetry, but is also found in prose
of all periods. It is named after the Greek letter chi, indicating a
'criss-cross' arrangement of terms.