sideloading was never a term used in any official documentation or communication the term was coined by the android community. we have to pick our own nose for that one.
On Android it specifically refers to installing something on your phone via your computer with the adb sideload command. If you just download and install an apk on the phone itself, it's not sideloading. For this reason it wouldn't make sense to use the term for installing anything on a PC.
It’s older than that, I remember using it to describe loading mp3s to iPods without using iTunes.