[plural form is Chuasmis]
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 ('Pleasure's a sin, and sometimes sin's a pleasure'—Byron), 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.