Antithesis
Etymology
From Greek "antithesis," combining "anti" (against) and "thesis" (a placing), literally "a setting against." It entered rhetoric as the deliberate juxtaposition of opposites — "not that I loved Caesar less, but that I loved Rome more."
noun · /an-TITH-uh-sis/
A direct opposite, or a rhetorical device that pairs contrasting ideas in balanced phrasing.
What it names
Names both the concept of a perfect opposite and the sentence structure that stages a contrast for effect.
Example
Their quiet new manager was the antithesis of the loud boss who came before.
Related idea
In dialectic, antithesis is the counter to a thesis; in rhetoric it is simply the artful pairing of opposites, no synthesis required.