Actually, in normal usage the word "sideloading" means transferring files laterally. That is to say, not in a client-server relationship.
In the world of android, "sideloading" became parlance for installing applications because historically you used adb on a computer to sideload the app on.
Did this change? Android-wise, I've always interpreted "sideload" as the adb method... just installing an apk in the UI isn't "sideloading", it's "install from apk" vs "install from play store".
In the world of android, "sideloading" became parlance for installing applications because historically you used adb on a computer to sideload the app on.