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".

This has not changed. It has always been considered sideloading to install an APK in the UI, as this is entirely identical to using adb.

Sideloading only means that you are providing the thing yourself, as opposed to using official channels.

Using F-Droid is considered sideloading.

You consider using F-Droid to be sideloading.

"sideloading" can mean many things.

In the domain you are used to, which isn't the only technical domain in existence.

Okay, Firefox is so big one could consider it a technical domain unto itself, but it's not so big its internal jargon overrides general usage.