A side-by-side contrast
If a waiter served you a whole fish and a scoop of chocolate ice cream on the same plate, your surprise might be caused by the juxtaposition, or the side-by-side contrast, of the two foods.
Any time unlike things bump up against each other, you can describe it as a juxtaposition. Imagine a funeral mourner telling jokes graveside, and you get the idea — the juxtaposition in this case is between grief and humor. Juxtaposition of two contrasting items is often done deliberately in writing, music, or art — in order to highlight their differences.
Thanks to Vocabulary.com