Why is it called Desktop Picture or Background in macOS and wallpaper in iOS? Is there a technical difference?
Mac continues the use of the decades-old analogy of the computer screen being your desktop.  The first GUI used in a mainstream computer, and almost every computer after that, was primarily for work use so that way of describing the screen, like a desktop with a large writing pad or family pictures, was the best way to describe the image.


PluckyVintage

iOS isn't a desktop computer, it's much more personal.  When iOS debuted, it was first on a small handheld phone.  You couldn't get much further away from a 'desktop' than an individual device you could customize.  There needed to be a way to refer to the background.  Because it could be customized and changed for each person's preference and tastes, the team at Apple decided on wallpaper.  

I can't find anything that definitively states why Apple decided to use that term but it fits their goal of a personal device.


genius.com

This is the way I've always viewed these two; I don't have any special insight or knowledge to say this way of explaining is the most accurate or better than anyone else's. We might not ever know why these decisions were made the way they were.