Chiasmus
Etymology
From Greek "chiasmos," "a crossing, an arrangement in the shape of the letter chi," from chiazein, "to mark with a chi," the X-shaped Greek letter. The name captures the visual crisscross of the pattern: the terms in the second clause cross back over those in the first. Rhetoricians have prized the figure since antiquity for its balance and bite.
noun · /ky-AZ-mus/
A rhetorical figure in which the second half of a phrase reverses the order of the first.
What it names
The mirrored crossing of “ask not what your country can do for you — ask what you can do for your country,” giving you a name for the reversal that makes a line stick.
Example
Her toast turned on a neat chiasmus: “We don’t stop playing because we grow old; we grow old because we stop playing.”
Related idea
A chiasmus reverses the order of ideas across two clauses; antithesis simply sets two opposed ideas against each other in parallel.