Antithesis will also work, although it's very formal. The OED Online's first definition for antithesis is
1. Rhetoric. An opposition or contrast of ideas, expressed by using as the corresponding members of two contiguous sentences or clauses,
words which are the opposites of, or strongly contrasted with, each
other; as ‘he must increase, but I must decrease,’ ‘in newness of
spirit, not in the oldness of the letter’.
One of the examples under that definition is
"In an Antithesis, one contrary must be pronounced louder than the other." (John Mason An essay on elocution, or, pronunciation 1748)
The third definition is also apropos:
- By extension: Direct or striking opposition of character or functions (between two things); contrast. Const. of, between (with
And one of its illustrations:
"The antithesis of natural and revealed religion." (Charles Kingsley ·
Alton Locke, tailor and poet II. xvii. 262 · 1st edition, 1850)
Apparently the word is from an ancient Greek term, via Latin; it was associated with Logic and Rhetoric all the way through.
Editing to add the second OED definition and my own paraphrase of the definition, per @Mazura's suggestion. I would phrase it as
Antithesis: A pair of opposing or contrasting ideas or things.
As the word in English has gone from a term of art to more general usage it has shifted away from this meaning so that now it's almost always used to mean the distinction between two things, or the second of two opposing things: where once we would have said X is antithetical to Y now we can say X is the antithesis of Y, and we're somewhat less likely to talk about the antithesis of X and Y.
But it certainly can still be used to describe the pair, e.g. X and Y are very different, and this antithesis is at the heart of... etc.